September 28th, 2018

“Overbore” — A Practical Definition by the Numbers

Overbore cartridge chart index ratio 50 BMG Case Capacity 30Br

What is “Overbore”? That’s a question rifle shooters can debate to no end. This article from our archives proposes one way to identify “overbore cartridges”. We think the approach outlined here is quite useful, but we know that there are other ways to define cartridges with “overbore” properties. Whenever we run this article, it stimulates a healthy debate among our readers — and that is probably a good thing.

Forum Member John L. has been intrigued by the question of “overbore” cartridges. People generally agree that overbore designs can be “barrel burners”, but is there a way to predict barrel life based on how radically a case is “overbore”? John notes that there is no generally accepted definition of “overbore”. Based on analyses of a wide variety of cartridges, John hoped to create a comparative index to determine whether a cartridge is more or less “overbore”. This, in turn, might help us predict barrel life and maybe even predict the cartridge’s accuracy potential.

John tells us: “I have read countless discussions about overbore cartridges for years. There seemed to be some widely accepted, general rules of thumb as to what makes a case ‘overbore’. In the simplest terms, a very big case pushing a relatively small diameter bullet is acknowledged as the classic overbore design. But it’s not just large powder capacity that creates an overbore situation — it is the relationship between powder capacity and barrel bore diameter. Looking at those two factors, we can express the ‘Overbore Index’ as a mathematical formula — the case capacity in grains of water divided by the area (in square inches) of the bore cross-section. This gives us an Index which lets us compare various cartridge designs.”

OVERBORE INDEX Chart

Overbore Index Chart

So what do these numbers mean? John says: “My own conclusion from much reading and analysis is that cartridges with case volume to bore area ratio less than 900 are most likely easy on barrels and those greater than 1000 are hard on barrels.” John acknowledges, however, that these numbers are just for comparison purposes. One can’t simply use the Index number, by itself, to predict barrel life. For example, one cannot conclude that a 600 Index number cartridge will necessarily give twice the barrel life of a 1200 Index cartridge. However, John says, a lower index number “seems to be a good predictor of barrel life”.

John’s system, while not perfect, does give us a benchmark to compare various cartridge designs. If, for example, you’re trying to decide between a 6.5-284 and a 260 Remington, it makes sense to compare the “Overbore Index” number for both cartridges. Then, of course, you have to consider other factors such as powder type, pressure, velocity, bullet weight, and barrel hardness.

Overbore Cases and Accuracy
Barrel life may not be the only thing predicted by the ratio of powder capacity to bore cross-section area. John thinks that if we look at our most accurate cartridges, such as the 6 PPC, and 30 BR, there’s some indication that lower Index numbers are associated with greater inherent accuracy. This is only a theory. John notes: “While I do not have the facilities to validate the hypothesis that the case capacity to bore area ratio is a good predictor of accuracy — along with other well-known factors — it seems to be one important factor.”

Permalink Bullets, Brass, Ammo, Tech Tip 1 Comment »
August 3rd, 2018

Can We Predict Useful Barrel Life? Insights from Dan Lilja

Lilja Rifle Barrels barrel life 3-groove AR15 Barrel heat

Barrel-maker Dan Lilja’s website has an excellent FAQ page that contains a wealth of useful information. On the Lilja FAQ Page as you’ll find informed answers to many commonly-asked questions. For example, Dan’s FAQ addresses the question of barrel life. Dan looks at factors that affect barrel longevity, and provides some predictions for barrel life, based on caliber, chambering, and intended use.

NOTE: This article was very well-received when it was first published last year. We are reprising it for the benefit of readers who missed it the first time.

Dan cautions that “Predicting barrel life is a complicated, highly variable subject — there is not a simple answer. Signs of accurate barrel life on the wane are increased copper fouling, lengthened throat depth, and decreased accuracy.” Dan also notes that barrels can wear prematurely from heat: “Any fast varmint-type cartridge can burn out a barrel in just a few hundred rounds if those rounds are shot one after another without letting the barrel cool between groups.”

Q. What Barrel Life, in number of rounds fired, can I expect from my new barrel?

A: That is a good question, asked often by our customers. But again there is not a simple answer. In my opinion there are two distinct types of barrel life. Accurate barrel life is probably the type most of us are referencing when we ask the question. But there is also absolute barrel life too. That is the point where a barrel will no longer stabilize a bullet and accuracy is wild. The benchrest shooter and to a lesser extent other target shooters are looking at accurate barrel life only when asking this question. To a benchrest shooter firing in matches where group size is the only measure of precision, accuracy is everything. But to a score shooter firing at a target, or bull, that is larger than the potential group size of the rifle, it is less important. And to the varmint hunter shooting prairie dog-size animals, the difference between a .25 MOA rifle or one that has dropped in accuracy to .5 MOA may not be noticeable in the field.

The big enemy to barrel life is heat. A barrel looses most of its accuracy due to erosion of the throat area of the barrel. Although wear on the crown from cleaning can cause problems too. The throat erosion is accelerated by heat. Any fast varmint-type cartridge can burn out a barrel in just a few hundred rounds if those rounds are shot one after another without letting the barrel cool between groups. A cartridge burning less powder will last longer or increasing the bore size for a given powder volume helps too. For example a .243 Winchester and a .308 Winchester both are based on the same case but the .308 will last longer because it has a larger bore.

And stainless steel barrels will last longer than chrome-moly barrels. This is due to the ability of stainless steel to resist heat erosion better than the chrome-moly steel.

Barrel Life Guidelines by Caliber and Cartridge Type
As a very rough rule of thumb I would say that with cartridges of .222 Remington size you could expect an accurate barrel life of 3000-4000 rounds. And varmint-type accuracy should be quite a bit longer than this.

For medium-size cartridges, such as the .308 Winchester, 7×57 and even the 25-06, 2000-3000 rounds of accurate life is reasonable.

Hot .224 caliber-type cartridges will not do as well, and 1000-2500 rounds is to be expected.

Bigger magnum hunting-type rounds will shoot from 1500-3000 accurate rounds. But the bigger 30-378 Weatherby types won’t do as well, being closer to the 1500-round figure.

These numbers are based on the use of stainless steel barrels. For chrome-moly barrels I would reduce these by roughly 20%.

The .17 and .50 calibers are rules unto themselves and I’m pressed to predict a figure.

The best life can be expected from the 22 long rifle (.22 LR) barrels with 5000-10,000 accurate rounds to be expected. We have in our shop one our drop-in Anschutz barrels that has 200,000 rounds through it and the shooter, a competitive small-bore shooter reported that it had just quit shooting.

Remember that predicting barrel life is a complicated, highly variable subject. You are the best judge of this with your particular barrel. Signs of accurate barrel life on the wane are increased copper fouling, lengthened throat depth, and decreased accuracy.

Lilja Rifle Barrels barrel life 3-groove AR15 Barrel heat

Benchrest Barrel Life — You May Be Surprised
I thought it might be interesting to point out a few exceptional Aggregates that I’ve fired with 6PPC benchrest rifles with barrels that had thousands of rounds through them. I know benchrest shooters that would never fire barrels with over 1500 shots fired in them in registered benchrest matches.

I fired my smallest 100-yard 5-shot Aggregate ever in 1992 at a registered benchrest match in Lewiston, Idaho. It was a .1558″ aggregate fired in the Heavy Varmint class. And that barrel had about 2100 rounds through it at the time.

Lilja Rifle Barrels barrel life 3-groove AR15 Barrel heat

Another good aggregate was fired at the 1997 NBRSA Nationals in Phoenix, Arizona during the 200-yard Light Varmint event. I placed second at this yardage with a 6PPC barrel that had over 2700 rounds through it at the time. I retired this barrel after that match because it had started to copper-foul quite a bit. But accuracy was still good.

Lilja Rifle Barrels barrel life 3-groove AR15 Barrel heat

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February 17th, 2018

Hexagonal Boron Nitride (HBN) for Bullet Coating

hexagonal Boron nitride bullets

For years, many shooters have coated bullets with Moly (molybdenum disulfide) or Danzac (tungsten disulfide or “WS2″). The idea was to reduce friction between bullets and barrel. In theory, this could lengthen barrel life and extend the number of rounds a shooter can fire between cleanings.

hexagonal Boron nitride bullets

Moly and WS2 both have their fans, but in recent years, many guys have switched to Hexagonal Boron Nitride (HBN), another dry lubricant. The advantage of HBN is that it won’t combine with moisture to create harmful acids. HBN is very slippery and it goes on clear, so it doesn’t leave a dirty mess on your hands or loading bench. Typically, HBN is applied via impact plating (tumbling), just as with Moly.

Good Source for Hexagonal Boron Nitride (HBN)
Paul Becigneul (aka PBike in our Forum) has been using HBN for many years with good results. He obtains his HBN from Momentive Performance Materials:

HBM boron nitride source momentive hexagonal bullet coatingMomentive Performance Materials
www.Momentive.com

Sales Contact: Robert Bell
Sales Email: robert.bell [at] momentive.com
Sales Phone: Robert Bell, (980) 231-5404

HBN Results — Both on Bullets and Barrel Bores
Many folks have asked, “Does Hexagonal Boron Nitride really work?” You’ll find answers to that and many other questions on gunsmith Stan Ware’s popular Bench-Talk.com Blog. There Paul Becigneul (aka Pbike) gives a detailed run-down on HBN use, comparing it to other friction-reducers. Paul also discusses the use of HBN in suspension to pre-coat the inside of barrels. Paul observes:

We coated our bullets … how we had been coating with WS2. Now our bullets have a slightly white sheen to them with kind of like a pearl coat. They are so slippery it takes a little practice to pick them up and not drop them on the trailer floor. What have we noticed down range? Nothing different from WS2 other than the black ring on your target around the bullet hole is now white or nonexistent. Our barrels clean just as clean as with WS2. Your hands aren’t black at the end of the day of shooting and that might be the most important part.

Interestingly, Becigneul decided to try a solution of HBN in alcohol, to pre-coat the inside of barrels. Paul had previously used a compound called Penephite to coat the inside of his barrels after cleaning. Paul explains:

If Penephite was used because it was slippery wouldn’t HBN be better? … We called Momentive again [our source for HBN], and talked about mixing HBN and 90% alcohol for a suspension agent to pre-lube our barrels. He though it sounded great but that the AC6111 Grade HBN would be better for this use. It would stand up in the alcohol suspension and cling to the barrel when passed through on a patch. We got some and mixed it in alcohol 90%. We use about one teaspoon in 16 ounces of alcohol.

We started using it this fall and what we have noticed is that now that first shot fired out of a clean and pre-lubed barrel can be trusted as the true impact point. We use tuners so now I got to the line, fire two shots judge my group for vertical, adjust the tuner as needed or not, and after tune has been achieved go to my record targets. This use has saved us in time at the bench and bullets in the backstop.

You really should read the whole article by Becigneul. He discusses the use of barrel lubes such as Penephite and “Lock-Ease” in some detail. Paul also provides links to HBN vendors and to the Material Safety Data Sheets (MSDS) for the various compounds he tested.

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January 26th, 2018

Toast in Six Seconds — The Brutal Truth of Short Barrel Life

Stopwatch barrel life

This thought-provoking article is one of the 15 most popular Daily Bulletin features over the last 12 months. We are republishing this story today for readers who may have missed it the first time around…

Here’s a little known fact that may startle most readers, even experienced gunsmiths: your barrel wears out in a matter of seconds. The useful life of a typical match barrel, in terms of actual bullet-in-barrel time, is only a few seconds. How can that be, you ask? Well you need to look at the actual time that bullets spend traveling through the bore during the barrel’s useful life. (Hint: it’s not very long).

Bullet-Time-in-Barrel Calculations
If a bullet flies at 3000 fps, it will pass through a 24″ (two-foot) barrel in 1/1500th of a second. If you have a useful barrel life of 3000 rounds, that would translate to just two seconds of actual bullet-in-barrel operating time.

Ah, but it’s not that simple. Your bullet starts at zero velocity and then accelerates as it passes through the bore, so the projectile’s average velocity is not the same as the 3000 fps muzzle velocity. So how long does a centerfire bullet (with 3000 fps MV) typically stay in the bore? The answer is about .002 seconds. This number was calculated by Varmint Al, who is a really smart engineer dude who worked at the Lawrence Livermore Laboratory, a government think tank that develops neutron bombs, fusion reactors and other simple stuff.

On his Barrel Tuner page, Varmint Al figured out that the amount of time a bullet spends in a barrel during firing is under .002 seconds. Al writes: “The approximate time that it takes a 3300 fps muzzle velocity bullet to exit the barrel, assuming a constant acceleration, is 0.0011 seconds. Actual exit times would be longer since the bullet is not under constant acceleration.”

We’ll use the .002 number for our calculations here, knowing that the exact number depends on barrel length and muzzle velocity. But .002 is a good average that errs, if anything, on the side of more barrel operating life rather than less.

So, if a bullet spends .002 seconds in the barrel during each shot, and you get 3000 rounds of accurate barrel life, how much actual firing time does the barrel deliver before it loses accuracy? That’s simple math: 3000 x .002 seconds = 6 seconds.

Stopwatch barrel lifeGone in Six Seconds. Want to Cry Now?
Six seconds. That’s how long your barrel actually functions (in terms of bullet-in-barrel shot time) before it “goes south”. Yes, we know some barrels last longer than 3000 rounds. On the other hand, plenty of .243 Win and 6.5-284 barrels lose accuracy in 1500 rounds or less. If your barrel loses accuracy at the 1500-round mark, then it only worked for three seconds! Of course, if you are shooting a “long-lived” .308 Win that goes 5000 rounds before losing accuracy, then you get a whopping TEN seconds of barrel life. Anyway you look at it, a rifle barrel has very little longevity, when you consider actual firing time.

People already lament the high cost of replacing barrels. Now that you know how short-lived barrels really are, you can complain even louder. Of course our analysis does give you even more of an excuse to buy a nice new Bartlein, Krieger, Shilen etc. barrel for that fine rifle of yours.

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May 20th, 2017

How to Prematurely Kill a Barrel — .243 Win Fast Firing Test

barrel life test rapid fire cooling

Can sustained rapid-fire shooting with no cool-down period wear out a quality barrel more quickly? The answer is “Yes” according to Forum member LCazador, who did an interesting comparison test with two .243 Winchester barrels. He started off with two, identical, match-grade HV taper stainless barrels. Both were NEW at the start of testing, and LCazador shot the same load through each: 95gr match bullets with 38 grains of Hodgdon Varget. After giving both barrels the same, gentle 20-round break-in, 300 rounds were then fired through each barrel — in very different ways. Barrel condition and wear were monitored with a borescope.

Barrel One — Slow Fire, Cool Down Periods, Cleaning Every 50 Rounds
At the end of the 300-round test, Barrel One looked brand new. There was none of the severe fire cracking found in Barrel Two. This barrel was shot no more than 10 times without a cool down and firing was done at a much slower pace. Cleaning for this barrel was done every 50 shots.

Barrel Two — Fast Firing, No Waiting, Cleaning Every 100 Rounds
The second barrel, which received hard use and minimal cleaning, was severely damaged with severe fire cracking at the leade and throat. As a result, the barrel had to be re-chambered. This barrel was shot 100 rounds at time without cleaning and was shot up to 20 times in succession without a cool down.

LESSON LEARNED — Heat Kills Barrel Life
Don’t let your barrel get too hot, and keep it clean. One afternoon can ruin a barrel!

Hawkeye Borescope imageMonitoring Barrel Wear with Borescope
Some folks worry too much about what their borescopes reveal — many barrels do not have to be “squeaky clean” to perform well. In fact some barrels run better after ten or more fouling shots. However, a borescope can be very helpful when your barrel starts losing accuracy for no apparent reason. Forum member FdShuster writes:

“A borescope is a positive way of backing up your suspicions when the rifle starts to throw an occasional (soon followed by more frequent) wild shot. Using the scope is also an excellent way to determine that the cause is barrel wear and not simply a need for a concentrated cleaning session to remove built up copper and more importantly, carbon fouling.

I’ve had a few barrels that gave every indication of being shot out. But I ‘scoped them out and found the cause to be nothing more than requiring a good cleaning. They then returned to their usual performance. There’s no guessing involved when you are able to get ‘up close and personal’ using the scope. The borescope also provides an excellent view of the all-important condition of the crown. My borescope is one of the most valuable investments I’ve ever made.”

Permalink Gunsmithing, Tech Tip 2 Comments »
February 26th, 2017

A Few Seconds at Best — The Short Operating Life of Barrels

Stopwatch barrel life

Here’s a little known fact that may startle most readers, even experienced gunsmiths: your barrel wears out in a matter of seconds. The useful life of a typical match barrel, in terms of actual bullet-in-barrel time, is only a few seconds. How can that be, you ask? Well you need to look at the actual time that bullets spend traveling through the bore during the barrel’s useful life. (Hint: it’s not very long).

Bullet-Time-in-Barrel Calculations
If a bullet flies at 3000 fps, it will pass through a 24″ (two-foot) barrel in 1/1500th of a second. If you have a useful barrel life of 3000 rounds, that would translate to just two seconds of actual bullet-in-barrel operating time.

Ah, but it’s not that simple. Your bullet starts at zero velocity and then accelerates as it passes through the bore, so the projectile’s average velocity is not the same as the 3000 fps muzzle velocity. So how long does a centerfire bullet (with 3000 fps MV) typically stay in the bore? The answer is about .002 seconds. This number was calculated by Varmint Al, who is a really smart engineer dude who worked at the Lawrence Livermore Laboratory, a government think tank that develops neutron bombs, fusion reactors and other simple stuff.

On his Barrel Tuner page, Varmint Al figured out that the amount of time a bullet spends in a barrel during firing is under .002 seconds. Al writes: “The approximate time that it takes a 3300 fps muzzle velocity bullet to exit the barrel, assuming a constant acceleration, is 0.0011 seconds. Actual exit times would be longer since the bullet is not under constant acceleration.”

We’ll use the .002 number for our calculations here, knowing that the exact number depends on barrel length and muzzle velocity. But .002 is a good average that errs, if anything, on the side of more barrel operating life rather than less.

So, if a bullet spends .002 seconds in the barrel during each shot, and you get 3000 rounds of accurate barrel life, how much actual firing time does the barrel deliver before it loses accuracy? That’s simple math: 3000 x .002 seconds = 6 seconds.

Stopwatch barrel lifeGone in Six Seconds. Want to Cry Now?
Six seconds. That’s how long your barrel actually functions (in terms of bullet-in-barrel shot time) before it “goes south”. Yes, we know some barrels last longer than 3000 rounds. On the other hand, plenty of .243 Win and 6.5-284 barrels lose accuracy in 1500 rounds or less. If your barrel loses accuracy at the 1500-round mark, then it only worked for three seconds! Of course, if you are shooting a “long-lived” .308 Win that goes 5000 rounds before losing accuracy, then you get a whopping TEN seconds of barrel life. Anyway you look at it, a rifle barrel has very little longevity, when you consider actual firing time.

People already lament the high cost of replacing barrels. Now that you know how short-lived barrels really are, you can complain even louder. Of course our analysis does give you even more of an excuse to buy a nice new Bartlein, Krieger, Shilen etc. barrel for that fine rifle of yours.

Permalink Gunsmithing, Tech Tip 2 Comments »
March 8th, 2016

Cost Per Round by Cartridge Type: .223, 6BR, 6XC, .308, 6.5-284

Shooting Cost by Cartridge Caliber type USAMU

Estimating Actual Cost per Round by Caliber
This article comes from the USAMU, which provide shooting and reloading tips on its Facebook Page. This week’s USAMU TECH TIP outlines a ballpark-estimate method of calculating the actual cost per round of different calibers. Some applications, and some shooters, by virtue of their high level of competition, require the very best ballistic performance available — “Darn the cost, full speed ahead!

If you are in serious contention to win a major competition, then losing even a single point to inferior ballistic performance could cost you a national title or record. However, this “horsepower” does come at a cost! Some calibers are barrel-burners, and some offer much longer barrel life. Look at this comparison chart:

Estimated Cost Per Round by Cartridge Type

Below are some estimated total expense per round (practice and competition) based on component costs, type used, expected barrel life and a standard, chambered barrel cost of $520.00 across calibers.

5.56x45mm: $0.46/round (barrel life 6,000 rounds)*

6mmBR: $0.81/round (barrel life 2800 rounds)

6XC: $0.97/round (barrel life 2200 rounds)

.308 Win: $0.80/round (barrel life 4500 rounds)

6.5-284: $1.24/round (barrel life 1100 rounds)

*Note the high round count estimate for 5.56x45mm. This is a bit deceptive, as it assumes a period of “lesser accuracy” use. The USAMU says: “Much of the difference you see here between 5.56 and .308 is due to using the 5.56 barrel for 100-200 yard training with less-expensive, 55gr Varmint bullets after its long-range utility is spent”.

Moreover, while some applications require specialized, high-cost components, others do not. And, if the shooter is still relatively new to the sport and hasn’t refined his skill to within the top few percentile of marksmen, a more economical caliber choice can help stretch a limited budget. Translation: More skill per dollar!

In this post, the prices for all items mentioned here were taken from a major component supplier’s current advertisements, and all brass was of top quality, except in the case of 5.56mm. There, 200 top-quality, imported cases were reserved for 600-yard shooting, and the other brass used was once-fired Lake City surplus.

Cartridge cases were assumed to be loaded 10 times each. [Your mileage may vary…] Bullet prices assumed the use of less-expensive, but good-quality match bullets for the bulk of shooting as appropriate.

The cost of top-tier, highly-expensive match bullets was also calculated for a realistic percentage of the shots fired, based on ones’ application. Barrel life by caliber was taken from likely estimates based on experience and good barrel maintenance.

Brass Costs Based on 10 Loads Per Case
Often, handloaders may calculate ammunition cost per round by adding the individual costs of primers, powder charges and projectiles. Many don’t consider the cost of brass, as it is reloaded several times. Here, we’ll consider the cost of enough top-quality brass to wear out a barrel in our given caliber, at 10 loads per case, except as noted above.

Don’t Forget Amortized Barrel Costs
Few shooters factor in the full, true cost of barrel life. Depending on caliber, that can dramatically increase the cost per round. For example, consider a long-range rifle in 6.5/284 caliber. This cartridge performs amazingly well, but at a cost. Ballpark estimated barrel life [in a top-quality barrel] is 1100 rounds. Some wear out faster, some last longer, but this gives a rough idea of what to expect.

Accurate barrels are a joy to use, but they are an expendable resource!
Shooting Cost by Cartridge Caliber type USAMU

A top-quality barrel plus installation was estimated at about $520.00. At 1100 rounds, barrel life adds $0.47 per round to our total cost. Thus, what had started out as an [components-only estimate, with brass cost] of $0.76/round now totals $1.24 per shot!

Cost Considerations When Choosing a Catridge Type
Some shooters might ask themselves if they could meet their present needs with a more economical caliber. If so, that equates to more practice and matches per available dollar, and more potential skill increase on the available budget.

Each shooter knows his skill level, practice needs, and shooting discipline’s requirements. Some might shoot NRA Service Rifle or Match Rifle using a 5.56mm with a long barrel life. Others might be Match Rifle shooters faced with choosing between, say, a 6mm BR vs. 6XC. A realistic assessment of ones needs, performance-wise, may help guide the shooter toward a caliber that’s most optimized to their needs at the moment.

Admittedly, the factors affecting cost for any individuals circumstances can vary significantly. However, hopefully this will provide one useful method of evaluating one’s training and competition choices, based on their skill, goals and needs.

USAMU reloading Facebook Page army tips tech

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December 30th, 2014

Life and Death of a Barrel — Can Barrel Useful Life Be Predicted?

We can predict, with some certainty, how long a light bulb will last (in use), or a shingle roof, or even a nuclear reactor. But how about barrels? Is there a way to reliably estimate barrel life based on known characteristics? This article explains one effort to quantify barrel life…

Rifle Barrel Life CalculatorHow long will a barrel last before the accuracy “goes south”? There are so many variables involved (powder type, bore diameter, bullet coatings etc.) that it’s hard to predict. You might say “Well, my buddy has a .243 and he got 1500 rounds before the throat was shot out” — those kind of comparisons can be useful, but they’re not very scientific, and they won’t help much if you’ve got a gun in a new chambering (such as the 6.5×47) for which long-term test results are lacking.

Is there a more reliable way to predict barrel life — one that will work for a broad range of calibers? Well, Forum member MikeCr has developed an Excel spreadsheet that accounts for a number of variables, and gives a pretty good estimate of useful barrel life, whether you’re shooting a .223 Rem or a 338 Lapua Magnum. Mike’s program predicts barrel life using five variables: 1) Bullet Diameter; 2) Powder Charge weight; 3) Powder Heat Potential (KJ/kg); 4) Pressure (in psi); and 5) Bullet Coating (yes/no). Mike provides a table with Heat Potential ratings for most popular powder types. The user needs to know the pressure of his load. This can be estimated with QuickLOAD.

You can download the lastest version of Mike’s spreadsheet below. You’ll need Excel or an Excel viewer to open the file.

Click to Download Spreadsheet: Barrel Life Spreadsheet (Latest Version)

Shown below is Mike’s Spreadsheet, with variables for a 6BR shooting 105gr “naked” bullets with 30.3 grains of Hodgdon Varget powder. The formula predicts 2401 rounds of barrel life. That corresponds pretty well to what we’d expect for a 6BR — about 2500 rounds.

Barrel Life ProgramBarrel Life Program

Mike observes: “There has been a lot of discussion lately related to cartridge design and resulting barrel life. This is a really important factor to consider amongst a myriad of choices. Barrel life is controversial, and subjective. There are no clear-cut standards for comparison. But a few years ago, I put together a spreadsheet based on Bart Bobbit’s rule of thumb. It worked pretty good, only occasionally failing some tests when validated against posted barrel lives.

According to Ken Howell, I had to account for pressure. And Henry Child’s powder temperature testing provided another piece needed. So, I’ve tweaked it here and there to pass more tests. From 223 Rem to 300 UltraMagnum. Another element added, but turned off, is shot interval. I would need way more tests to lock in on this. But everyone knows, the faster you shoot, the worse the barrel life.

Anyway, another factor hard to define is ‘accurate’ barrel life. This cannot be quantified without standards. Barrels are replaced when expectations are no longer met. I feel that a [barrel] passes peak potential in a finite period due to throat erosion. But that don’t mean it’s toast, if it still shoots well enough. It’s just as likely that many of us never see that peak potential anyway. It’s a slippery thing. Point-blank BR competitors will toss a barrel when it leaves the 1s. I could get another 4000 rounds from it, and be content with its performance, I’m sure.”

NOTE: Mike says: “This spreadsheet may show a lower barrel life than you prefer. But it pretty well spotlights cartridges to stay away from if you plan much time at the range or in dog town.”

Editor’s Comment: Mike’s spreadsheet is a helpful tool, but it is NOT a definitive “take-it-to-the-bank” indicator of barrel life. Mike cautions that predicting barrel life involves so many different factors (including how hot the barrel is run), that the task is a bit like predicting tread life on car tires. Still, the spreadsheet is very helpful. It can certainly warn us that some chamberings (such as the 6-284) are likely to be barrel burners. That can help you make a smart decision when choosing a chambering for your next rifle.

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March 2nd, 2014

Defining “Overbore” Cartridges via Comparative Index

What is “Overbore”? That’s a question rifle shooters can debate to no end. This article from our archives proposes one way to identify “overbore cartridges”. We think the approach outlined here is quite useful, but we know that there are other ways to define cartridges with “overbore” properties. Whenever we run this article, it stimulates a healthy debate among our readers — and that is probably a good thing.

Forum Member John L. has been intrigued by the question of “overbore” cartridges. People generally agree that overbore designs can be “barrel burners”, but is there a way to predict barrel life based on how radically a case is “overbore”? John notes that there is no generally accepted definition of “overbore”. Based on analyses of a wide variety of cartridges, John hoped to create a comparative index to determine whether a cartridge is more or less “overbore”. This, in turn, might help us predict barrel life and maybe even predict the cartridge’s accuracy potential.

John tells us: “I have read countless discussions about overbore cartridges for years. There seemed to be some widely accepted, general rules of thumb as to what makes a case ‘overbore’. In the simplest terms, a very big case pushing a relatively small diameter bullet is acknowledged as the classic overbore design. But it’s not just large powder capacity that creates an overbore situation — it is the relationship between powder capacity and barrel bore diameter. Looking at those two factors, we can express the ‘Overbore Index’ as a mathematical formula — the case capacity in grains of water divided by the area (in square inches) of the bore cross-section. This gives us an Index which lets us compare various cartridge designs.”

OVERBORE INDEX Chart

Overbore Index Chart

So what do these numbers mean? John says: “My own conclusion from much reading and analysis is that cartridges with case volume to bore area ratio less than 900 are most likely easy on barrels and those greater than 1000 are hard on barrels.” John acknowledges, however, that these numbers are just for comparison purposes. One can’t simply use the Index number, by itself, to predict barrel life. For example, one cannot conclude that a 600 Index number cartridge will necessarily give twice the barrel life of a 1200 Index cartridge. However, John says, a lower index number “seems to be a good predictor of barrel life”.

John’s system, while not perfect, does give us a benchmark to compare various cartridge designs. If, for example, you’re trying to decide between a 6.5-284 and a 260 Remington, it makes sense to compare the “Overbore Index” number for both cartridges. Then, of course, you have to consider other factors such as powder type, pressure, velocity, bullet weight, and barrel hardness.

Overbore Cases and Accuracy
Barrel life may not be the only thing predicted by the ratio of powder capacity to bore cross-section area. John thinks that if we look at our most accurate cartridges, such as the 6 PPC, and 30 BR, there’s some indication that lower Index numbers are associated with greater inherent accuracy. This is only a theory. John notes: “While I do not have the facilities to validate the hypothesis that the case capacity to bore area ratio is a good predictor of accuracy — along with other well-known factors — it seems to be one important factor.”

Permalink Bullets, Brass, Ammo, Reloading 26 Comments »
April 25th, 2013

What is an “Overbore” Cartridge? Let’s Look at Some Numbers…

Forum Member John L. has been intrigued by the question of “overbore” cartridges. People generally agree that overbore designs can be “barrel burners”, but is there a way to predict barrel life based on how radically a case is “overbore”? John notes that there is no generally accepted definition of “overbore”. Based on analyses of a wide variety of cartridges, John hoped to create a comparative index to determine whether a cartridge is more or less “overbore”. This, in turn, might help us predict barrel life and maybe even predict the cartridge’s accuracy potential.

John tells us: “I have read countless discussions about overbore cartridges for years. There seemed to be some widely accepted, general rules of thumb as to what makes a case ‘overbore’. In the simplest terms, a very big case pushing a relatively small diameter bullet is acknowledged as the classic overbore design. But it’s not just large powder capacity that creates an overbore situation — it is the relationship between powder capacity and barrel bore diameter. Looking at those two factors, we can express the ‘Overbore Index’ as a mathematical formula — the case capacity in grains of water divided by the area (in square inches) of the bore cross-section. This gives us an Index which lets us compare various cartridge designs.”

OVERBORE INDEX Chart

Overbore Index Chart

So what do these numbers mean? John says: “My own conclusion from much reading and analysis is that cartridges with case volume to bore area ratio less than 900 are most likely easy on barrels and those greater than 1000 are hard on barrels.” John acknowledges, however, that these numbers are just for comparison purposes. One can’t simply use the Index number, by itself, to predict barrel life. For example, one cannot conclude that a 600 Index number cartridge will necessarily give twice the barrel life of a 1200 Index cartridge. However, John says, a lower index number “seems to be a good predictor of barrel life”.

John’s system, while not perfect, does give us a benchmark to compare various cartridge designs. If, for example, you’re trying to decide between a 6.5-284 and a 260 Remington, it makes sense to compare the “Overbore Index” number for both cartridges. Then, of course, you have to consider other factors such as powder type, pressure, velocity, bullet weight, and barrel hardness.

Overbore Cases and Accuracy
Barrel life may not be the only thing predicted by the ratio of powder capacity to bore cross-section area. John thinks that if we look at our most accurate cartridges, such as the 6 PPC, and 30 BR, there’s some indication that lower Index numbers are associated with greater inherent accuracy. This is only a theory. John notes: “While I do not have the facilities to validate the hypothesis that the case capacity to bore area ratio is a good predictor of accuracy — along with other well-known factors — it seems to be one important factor.”

Permalink - Articles, Bullets, Brass, Ammo 7 Comments »
July 27th, 2012

Handy Excel Formula Predicts Useful Barrel Life

Rifle Barrel Life CalculatorHow long will a barrel last before the accuracy “goes south”? There are so many variables involved (powder type, bore diameter, bullet coatings etc.) that it’s hard to predict. You might say “Well, my buddy has a .243 and he got 1500 rounds before the throat was shot out” — those kind of comparisons can be useful, but they’re not very scientific, and they won’t help much if you’ve got a gun in a new chambering (such as the 6.5×47) for which long-term test results are lacking.

Is there a more reliable way to predict barrel life — one that will work for a broad range of calibers? Well, Forum member MikeCr has developed an Excel spreadsheet that accounts for a number of variables, and gives a pretty good estimate of useful barrel life, whether you’re shooting a .223 Rem or a 338 Lapua Magnum. Mike’s program predicts barrel life using five variables: 1) Bullet Diameter; 2) Powder Charge weight; 3) Powder Heat Potential (KJ/kg); 4) Pressure (in psi); and 5) Bullet Coating (yes/no). Mike provides a table with Heat Potential ratings for most popular powder types. The user needs to know the pressure of his load. This can be estimated with QuickLOAD.

You can download the lastest version of Mike’s spreadsheet below. You’ll need Excel or an Excel viewer to open the file.

Click to Download Spreadsheet: Barrel Life Spreadsheet (Latest Version)

Shown below is Mike’s Spreadsheet, with variables for a 6BR shooting 105gr “naked” bullets with 30.3 grains of Hodgdon Varget powder. The formula predicts 2401 rounds of barrel life. That corresponds pretty well to what we’d expect for a 6BR — about 2500 rounds.

Barrel Life ProgramBarrel Life Program

Mike observes: “There has been alot of discussion lately related to cartridge design and resulting barrel life. This is a really important factor to consider amongst a myriad of choices. Barrel life is controversial, and subjective. There are no clear-cut standards for comparison. But a few years ago, I put together a spreadsheet based on Bart Bobbit’s rule of thumb. It worked pretty good, only occasionally failing some tests when validated against posted barrel lives.

According to Ken Howell, I had to account for pressure. And Henry Child’s powder temperature testing provided another piece needed. So, I’ve tweaked it here and there to pass more tests. From 223rem to 300 UltraMagnum. Another element added, but turned off is shot interval. I would need way more tests to lock in on this. But everyone knows, the faster you shoot, the worse the barrel life.

Anyway, another factor hard to define is ‘accurate’ barrel life. This cannot be quantified without standards. Barrels are replaced when expectations are no longer met. I feel that a [barrel] passes peak potential in a finite period due to throat erosion. But that don’t mean it’s toast, if it still shoots well enough. It’s just as likely that many of us never see that peak potential anyway. It’s a slippery thing. Point-blank BR competitors will toss a barrel when it leaves the 1s. I could get another 4000 rounds from it, and be content with its performance, I’m sure.”

NOTE: Mike says: “This spreadsheet may show a lower barrel life than you prefer. But it pretty well spotlights cartridges to stay away from if you plan much time at the range or in dog town.”

Editor’s Comment: We want to stress that Mike’s spreadsheet is a helpful tool, but it is not a definitive “take-it-to-the-bank” indicator of barrel life. Mike cautions that predicting barrel life involves so many different factors (including how hot the barrel is run), that the task is a bit like predicting tread life on car tires. Still, the spreadsheet is very helpful. It can certainly put shooters on notice that some chamberings (such as the 6-284) are likely to be a barrel burners. That can help you make a smart decision when choosing a chambering for your next rifle.

Permalink Gunsmithing, Tech Tip 18 Comments »
May 20th, 2010

Is the 6.5×47 Lapua the Next, Great Do-It-All Cartridge?

6.5x47 LapuaLapua developed the 6.5×47 Lapua cartridge for International 300m competition. Lapua wanted a cartridge that could match the “pure accuracy” of the 6mmBR, but with even better ballistics and good barrel life. The 6.5x47L is now really coming into its own. In the hands of NBRSA long-range Hall of Famer Don Nielson, the 6.5x47L has won two NBRSA 600-yard Nationals convincingly. The cartridge is winning Varmint Silhouette matches, and Tactical competitors are finding the cartridge delivers great accuracy with much less recoil than a .308 — plus it feeds well from magazines. With advanced powders such as Alliant’s Reloder 17, the 6.5x47L can deliver surprising velocities, even with the heavy 139-142 grain bullets, though we still think the 130-grainers may be optimal for the cartridge. Don Nielson used Berger 130gr bullets “right out of the box” to win the NBRSA 600-yard Nationals, setting records in the process. In addition to its paper-punching abilities, the 6.5×47 Lapua is a capable hunting cartridge, delivering velocities that approach a .260 Remington with 120-130 grain projectiles. Considering all this — is the 6.5×47 Lapua the next, great do-it-all cartridge — a chambering that can win a benchrest match one weekend and harvest a whitetail the next?

6.5x47 Lapua

6.5×47 Lapua Ballistics Chart | 6.5×47 Lapua Cartridge Diagram (PDF)

6.5×47 Lapua Is a Hot Topic on Our Forum
In our AccurateShooter Forum, there has been a thread discussing whether the 6.5x47L or 6mmBR is better for the 600-yard game. Forum member Lloyd (aka “1Shot”) wrote:

How many years have the 6BR and its variants been in existence as opposed to the 6.5x47L? I pose this question to you because we all know that with time and experience, comes knowledge. In a short period of time, (relatively speaking) the 6.5x47L has made great strides. I’ll make a bet that the 6.5x47L will outshoot the 6BR within the same time frame of its existence. It will just take time and shooters like Sam Hall to bring this cartridge to its full potential.”

Responding to Lloyd’s post, British gun writer Laurie Holland analyzed the present and possible future of the 6.5x47L cartridge. Laurie makes some very interesting points, considering the role the 6.5x47L may play in F-class competition, and in 300 to 600-yard benchrest. Laurie’s post is worth a read….

6.5x47 LapuaLaurie Holland Talks About the 6.5×47 Lapua
“Lloyd, you may well be right on this. There is that intangible something issue though that sees one cartridge become ‘great’ in a shooting field, while others that should theoretically compete somehow never quite get there. The example that comes to mind is the PPC in short range BR of course[.] The PPC outshoots similar rivals, and we don’t know why. At the moment, the 6BR and BRX/Dasher variants look like they may be getting to the same point in 300 to 600-yard BR type competition. That’s not to say they won’t be pushed out of the top spot by something better, or even matched by a rival, but it looks like it won’t happen quickly or soon.

One thing people often forget is that BR can see 5 shots rattled off in under 30 seconds while the wind conditions hold. F-Class or any other form of deliberate, marked-target shooting is different because of the enforced gap between shots. This is particularly so on our side of the Atlantic (or north of the 49th Parallel). Unlike your F-Class, Fullbore, Palma, CLRP etc. shooting, we British Commonwealth types are squadded two to a target (sometimes up to four in local shoots) and there is therefore a minimum three or so minutes between taking each shot by an individual shooter. Fairly typically, I shot in a club 600-yard F-Class comp last Sunday and found myself with two others on ‘my’ target. Throw in one of them missing with both sighters and the resulting delays while the RO had to be asked for the target to be pulled and checked and people looking for his subsequent fall of shot and it took around 70-75 minutes to get through 2 sighters and 20 score shots. So each shot is virtually a new start so far as the wind-call goes.

The 6.5X47L likely gives a significant benefit even at 600 yards over the 6mmBR thanks to the external ballistics improvements in such a scenario. What has to be seen is how it stands up in national level competition against the big sevens since that’s the competition in F-Open. Until this year, all national GB F-Class Association rounds were shot over a mixture of 800, 900, 1000-yard matches with the emphasis on the 1000. The 6.5X47L can’t hack it against 180gr 7mm Berger VLDs at 3,200 fps in these conditions (nor could the US F-Class Team’s 6.5-284 Norma in last summer’s F-Class World Championship at Bisley). Club / regional level may turn out different with a bigger mix of ranges (distances) in a season’s programme. And (here’s the killer) — the 7mmWSM brigade can’t afford to shoot say 15 such matches in a season given the barrel life issue. 15 matches is 300-330 shots, or 50% of barrel life. So they enter one or two matches only for practice, checking sight-settings etc. Unless you do your own gunsmithing, barrel replacement by a top gunsmith with a Bartlein or Krieger tube is an expensive job here — a bit over £700 all in which is $1,000 + in translation[.]

So, accurate and ballistically-efficient smaller cartridges that give long barrel life have a bright future in European and British Commonwealth shooting, and I think the 6.5X47L is going to be a key player in this role. Also, as in the USA, many people want a multi-purpose longarm, and this cartridge is an excellent long-range fox/crow round and ideal for most of our deer species too.

To go back to the very original question in this thread, if I were having a multi-discipline target shooting single-shot rifle built for club / regional competition shooting, I’d stick to 6BR or variants if it were primarily for 600-yard or shorter range competitions, but I’d go for 6.5X47L if there were 800 to 1,000-yard matches in the mix. If it were for F-Class at national level, I’d have neither, but stick to .308W in the F/TR division which I shoot now. — Laurie, York, England”

Permalink Competition, Reloading 3 Comments »