July 5th, 2019

Optimize Bullet RPM with Berger Twist Rate Stability Calculator

Berger twist rate calculator

Berger twist rate calculatorBerger Twist-Rate Stability Calculator
On the Berger Bullets website you’ll find a handy Twist-Rate Stability Calculator that predicts your gyroscopic stability factor (SG) based on mulitiple variables: velocity, bullet length, bullet weight, barrel twist rate, ambient temperature, and altitude. This cool tool tells you if your chosen bullet will really stabilize in your barrel.


CLICK HERE to Go to TWIST RATE CALCULATOR PAGE »

How to Use Berger’s Twist Rate Calculator
Using the Twist Rate Calculator is simple. Just enter the bullet DIAMETER (e.g. .264), bullet WEIGHT (in grains), and bullet overall LENGTH (in inches). On its website, Berger conveniently provides this info for all its bullet types. For other brands, we suggest you weigh three examples of your chosen bullet, and also measure the length on three samples. Then use the average weight and length of the three. To calculate bullet stability, simply enter your bullet data (along with observed Muzzle Velocity, outside Temperature, and Altitude) and click “Calculate SG”. Try different twist rate numbers (and recalculate) until you get an SG value of 1.4 (or higher).

Gyroscopic Stability (SG) and Twist Rate
Berger’s Twist Rate Calculator provides a predicted stability value called “SG” (for “Gyroscopic Stability”). This indicates the Gyroscopic Stability applied to the bullet by spin. This number is derived from the basic equation: SG = (rigidity of the spinning mass)/(overturning aerodynamic torque).

Berger twist rate calculator

If you have an SG under 1.0, your bullet is predicted not to stabilize. If you have between 1.0 and 1.1 SG, your bullet may or may not stabilize. If you have an SG greater than 1.1, your bullet should stabilize under optimal conditions, but stabilization might not be adequate when temperature, altitude, or other variables are less-than-optimal. That’s why Berger normally recommends at least 1.5 SG to get out of the “Marginal Stability” zone.

In his book Applied Ballistics For Long-Range Shooting, Bryan Litz (Berger Ballistician) recommends at least a 1.4 SG rating when selecting a barrel twist for a particular bullet. This gives you a safety margin for shooting under various conditions, such as higher or lower altitudes or temperatures.

Story idea from EdLongrange. We welcome reader submissions.
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June 10th, 2019

Tech Tip: Figuring Out Barrel Twist Rates

FirearmsID.com barrel rifling diagram
Erik Dahlberg illustration courtesy FireArmsID.com.

Sometimes you’ll get a barrel that doesn’t stabilize bullets the way you’d anticipate, based on the stated (or presumed) twist rate. A barrel might have 1:10″ stamped on the side but it is, in truth, a 1:10.5″ twist or even a 1:9.5″. Cut-rifled barrels, such as Kriegers and Bartleins, normally hold very true to the specified twist rate. With buttoned barrels, due to the nature of the rifling process, there’s a greater chance of a small variation in twist rate. And yes, factory barrels can be slightly out of spec as well.

After buying a new barrel, you should determine the true twist rate BEFORE you start load development. You don’t want to invest in a large supply of expensive bullets only to find that that won’t stabilize because your “8 twist” barrel is really a 1:8.5″. Sinclair International provides a simple procedure for determining the actual twist rate of your barrel.

Sinclair’s Simple Twist Rate Measurement Method
If are unsure of the twist rate of the barrel, you can measure it yourself in a couple of minutes. You need a good cleaning rod with a rotating handle and a jag with a fairly tight fitting patch. Utilize a rod guide if you are accessing the barrel through the breech or a muzzle guide if you are going to come in from the muzzle end. Make sure the rod rotates freely in the handle under load. Start the patch into the barrel for a few inches and then stop. Put a piece of tape at the back of the rod by the handle (like a flag) or mark the rod in some way. Measure how much of the rod is still protruding from the rod guide. You can either measure from the rod guide or muzzle guide back to the flag or to a spot on the handle. Next, continue to push the rod in until the mark or tape flag has made one complete revolution. Re-measure the amount of rod that is left sticking out of the barrel. Use the same reference marks as you did on the first measurement. Next, subtract this measurement from the first measurement. This number is the twist rate. For example, if the rod has 24 inches remaining at the start and 16 inches remain after making one revolution, you have 8 inches of travel, thus a 1:8 twist barrel.

Determining Barrel Twist Rate Empirically
Twist rate is defined as the distance in inches of barrel that the rifling takes to make one complete revolution. An example would be a 1:10″ twist rate. A 1:10″ barrel has rifling that makes one complete revolution in 10 inches of barrel length. Rifle manufacturers usually publish twist rates for their standard rifle offerings and custom barrels are always ordered by caliber, contour, and twist rate. If you are having a custom barrel chambered you can ask the gunsmith to mark the barrel with the twist rate.

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January 24th, 2019

How to Calculate Bullet RPM — Spin Rates and Stability

Spin rate stability bullet speed RPM Formula stabilization barrel twist
Photo by Werner Mehl, www.kurzzeit.com, all rights reserved.

Most serious shooters can tell you the muzzle velocity (MV) of their ammunition, based on measurements taken with a chronograph, or listed from a manufacturer’s data sheet. (Of course, actual speed tests conducted with YOUR gun will be more reliable.)

Bullet RPM = MV X 720/Twist Rate (in inches)

However, if you ask a typical reloader for the rotational rate of his bullet, in revolutions per minute (RPM), chances are he can’t give you an answer.

Knowing the true spin rate or RPM of your bullets is very important. First, spin rate, or RPM, will dramatically affect the performance of a bullet on a game animal. Ask any varminter and he’ll tell you that ultra-high RPM produces more dramatic hits with more “varmint hang time”. Second, RPM is important for bullet integrity. If you spin your bullets too fast, this heats up the jackets and also increases the centrifugal force acting on the jacket, pulling it outward. The combination of heat, friction, and centrifugal force can cause jacket failure and bullet “blow-ups” if you spin your bullets too fast.

Accuracy and RPM
Additionally, bullet RPM is very important for accuracy. Nearly all modern rifles use spin-stablized bullets. The barrel’s rifling imparts spin to the bullet as it passes through the bore. This rotation stabilizes the bullet in flight. Different bullets need different spin rates to perform optimally. Generally speaking, among bullets of the same caliber, longer bullets need more RPM to stabilize than do shorter bullets–often a lot more RPM.

It is generally believed that, for match bullets, best accuracy is achieved at the minimal spin rates that will fully stabilize the particular bullet at the distances where the bullet must perform. That’s why short-range 6PPC benchrest shooters use relatively slow twist rates, such as 1:14″, to stabilize their short, flatbase bullets. They could use “fast” twist rates such as 1:8″, but this delivers more bullet RPM than necessary. Match results have demonstrated conclusively that the slower twist rates produce better accuracy with these bullets.

On the other hand, Research by Bryan Litz of Applied Ballistics has shown that with long, boat-tailed bullets, best accuracy may be achieved with twist rates slightly “faster” than the minimum required for stabilization. The reasons for this are somewhat complex — but it’s something to consider when you buy your next barrel. If, for example, the bullet-maker recommends a 1:8.25″ twist, you might want to get a true 1:8″-twist barrel.

Calculating Bullet RPM from MV and Twist Rate
The lesson here is that you want to use the optimal RPM for each bullet type. So how do you calculate that? Bullet RPM is a function of two factors, barrel twist rate and velocity through the bore. With a given rifling twist rate, the quicker the bullet passes through the rifling, the faster it will be spinning when it leaves the muzzle. To a certain extent, then, if you speed up the bullet, you can use a slower twist rate, and still end up with enough RPM to stabilize the bullet. But you have to know how to calculate RPM so you can maintain sufficient revs.

Bullet RPM Formula
Here is a simple formula for calculating bullet RPM:

MV x (12/twist rate in inches) x 60 = Bullet RPM

Quick Version: MV X 720/Twist Rate = RPM

Example One: In a 1:12″ twist barrel the bullet will make one complete revolution for every 12″ (or 1 foot) it travels through the bore. This makes the RPM calculation very easy. With a velocity of 3000 feet per second (FPS), in a 1:12″ twist barrel, the bullet will spin 3000 revolutions per SECOND (because it is traveling exactly one foot, and thereby making one complete revolution, in 1/3000 of a second). To convert to RPM, simply multiply by 60 since there are 60 seconds in a minute. Thus, at 3000 FPS, a bullet will be spinning at 3000 x 60, or 180,000 RPM, when it leaves the barrel.

Example Two: What about a faster twist rate, say a 1:8″ twist? We know the bullet will be spinning faster than in Example One, but how much faster? Using the formula, this is simple to calculate. Assuming the same MV of 3000 FPS, the bullet makes 12/8 or 1.5 revolutions for each 12″ or one foot it travels in the bore. Accordingly, the RPM is 3000 x (12/8) x 60, or 270,000 RPM.

Implications for Gun Builders and Reloaders
Calculating the RPM based on twist rate and MV gives us some very important information. Number one, we can tailor the load to decrease velocity just enough to avoid jacket failure and bullet blow-up at excessive RPMs. Number two, knowing how to find bullet RPM helps us compare barrels of different twist rates. Once we find that a bullet is stable at a given RPM, that gives us a “target” to meet or exceed in other barrels with a different twist rate. Although there are other important factors to consider, if you speed up the bullet (i.e. increase MV), you MAY be able to run a slower twist-rate barrel, so long as you maintain the requisite RPM for stabilization and other factors contributing to Gyroscopic Stability are present. In fact, you may need somewhat MORE RPM as you increase velocity, because more speed puts more pressure, a destabilizing force, on the nose of the bullet. You need to compensate for that destabilizing force with somewhat more RPM. But, as a general rule, if you increase velocity you CAN decrease twist rate. What’s the benefit? The slower twist-rate barrel may, potentially, be more accurate. And barrel heat and friction may be reduced somewhat.

Just remember that as you reduce twist rate you need to increase velocity, and you may need somewhat MORE RPM than before. (As velocities climb, destabilizing forces increase somewhat, RPM being equal.) There is a formula by Don Miller that can help you calculate how much you can slow down the twist rate as you increase velocity.

CLICK HERE for Miller Formula in Excel Spreadsheet Format

That said, we note that bullet-makers provide a recommended twist rate for their bullets. This is the “safe bet” to achieve stabilization with that bullet, and it may also indicate the twist rate at which the bullet shoots best. Though the RPM number alone does not assure gyroscopic stability, an RPM-based calculation can be very useful. We’ve seen real world examples where a bullet that needs an 8-twist barrel at 2800 FPS MV, would stabilize in a 9-twist barrel at 3200 FPS MV. Consider these examples.

MV = 2800 FPS
8-Twist RPM = 2800 x (12/8) x 60 = 252,000 RPM

MV = 3200 FPS
9-Twist RPM = 3200 x (12/9) x 60 = 256,000 RPM

Of course max velocity will be limited by case capacity and pressure. You can’t switch to a slower twist-rate barrel and maintain RPM if you’ve already maxed out your MV. But the Miller Formula can help you select an optimal twist rate if you’re thinking of running the same bullet in a larger case with more potential velocity.

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July 31st, 2017

Choosing Optimal Barrel Twist Rate — Tips from Glen Zediker

Glen Zediker Twist Rate .223 Rem Barrel Top Grade Ammo Midsouth
Here’s an extreme range of .224-Caliber bullets: 35gr varmint bullet and 90gr match bullet. Of course, along with bullet length/design, you need to consider MV when choosing twist rate.

Even with the same caliber (and same bullet weight), different bullet types may require different rates of spin to stabilize properly. The bullet’s initial spin rate (RPM) is a function of the bullet’s muzzle velocity and the spin imparted by the rifling in the barrel. You want to ensure your bullet is stable throughout flight. It is better to have too much spin than too little, according to many ballistics experts, including Bryan Litz of Applied Ballistics. Glen Zediker has some basic tips concerning barrel twist rates and bullet stability. These come from his latest book, Top Grade Ammo.

Choosing the Right Twist Rate
I’d always rather have a twist too fast than not fast enough. Generally… I recommend erring toward the faster side of a barrel twist decision. 1:8″ twist is becoming a “new standard” for .224 caliber, replacing 1:9″ in the process. The reason is that new bullets tend to be bigger rather than smaller. Don’t let a too-slow twist limit your capacity to [achieve] better long-range performance.

Base your next barrel twist rate decision on the longest, heaviest bullets you choose to use, and at the same time realize that the rate you choose will in turn limit your bullet choices. If the longest, heaviest bullet you’ll shoot (ever) is a 55-grain .224, then there’s honestly no reason not to use a 1:12″. Likewise true for .308-caliber: unless you’re going over 200-grain bullet weight, a 1:10″ will perform perfectly well.

Glen Zediker Twist Rate .223 Rem Barrel Top Grade Ammo Midsouth

Bullet Length is More Critical than Weight
Bullet length, not weight, [primarily] determines how much rotation is necessary for stability. Twist rate suggestions, though, are most usually given with respect to bullet weight, but that’s more of a generality for convenience’s sake, I think. The reason is that with the introduction of higher-ballistic-coefficient bullet designs, which are longer than conventional forms, it is easily possible to have two same-weight bullets that won’t both stabilize from the same twist rate.

Evidence of Instability
The tell-tale for an unstable (wobbling or tumbling) bullet is an oblong hole in the target paper, a “keyhole,” and that means the bullet contacted the target at some attitude other than nose-first.

Glen Zediker Twist Rate .223 Rem Barrel Top Grade Ammo MidsouthIncreasing Barrel Length Can Deliver More Velocity, But That May Still Not Provide Enough Stability if the Twist Rate Is Too Slow
Bullet speed and barrel length have an influence on bullet stability, and a higher muzzle velocity through a longer tube will bring on more effect from the twist, but it’s a little too edgy if a particular bullet stabilizes only when running maximum velocity.

My failed 90-grain .224 experiment is a good example of that: I could get them asleep in a 1:7″ twist, 25-inch barrel, which was chambered in .22 PPC, but could not get them stabilized in a 20-inch 1:7″ .223 Rem. The answer always is to get a twist that’s correct.

These tips were adapted from Glen’s newest book, Top-Grade Ammo, available at Midsouth. To learn more about this book and other Zediker titles, and read a host of downloadable articles, visit ZedikerPublishing.com.

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August 5th, 2016

Twist Rate: Common Misconceptions about Twist and Stabilization

FirearmsID.com barrel rifling diagram

Understanding Twist: Bullet Stabilization

by Sierra Bullets Ballistic Technician Paul Box for Sierra Bullets Blog.

Based on the questions we get on a daily basis on our 800 (Customer Support) line, twist is one of the most misunderstood subjects in the gun field. So let’s look deeper into this mystery and get a better understanding of what twist really means.

When you see the term 1:14″ (1-14) or 1:9″ twist, just exactly what does this mean? A rifle having a 1:14″ twist means the bullet will rotate one complete revolution every fourteen inches of the barrel. Naturally a 1:9″ turns one time every nine inches that it travels down the barrel. Now, here’s something that some people have trouble with. I’ve had calls from shooters thinking that a 1:14″ twist was faster than a 1:9″ because the number was higher with the 1:14″. The easiest way to remember this is the higher the number, the slower the twist rate is.

Now, the biggest misconception is that if a shooter has a .223 with a 1:8″ twist, his rifle won’t stabilize a 55gr bullet or anything lighter. So let’s look at what is required. The longer a bullet is for its diameter, the faster the twist has to be to stabilize it. In the case of the .223 with a 1:8″ twist, this was designed to stabilize 80gr bullets in this diameter. In truth the opposite is true. A 1:8″ will spin a 55gr faster than what is required in order to stabilize that length of bullet. If you have a bullet with good concentricity in its jacket, over-spinning it will not [normally] hurt its accuracy potential. [Editor’s Note: In addition, the faster twist rate will not, normally, decrease velocity significantly. That’s been confirmed by testing done by Bryan Litz’s Applied Ballistics Labs. There may be some minor speed loss.]

FirearmsID.com barrel rifling diagram
Many barrel-makers mark the twist rate and bore dimensions on their barrel blanks.

Think of it like tires on your truck. If you have a new set of tires put on your truck, and they balance them proper at the tire shop, you can drive down a street in town at 35 MPH and they spin perfect. You can get out on the highway and drive 65 MPH and they still spin perfect. A bullet acts the same way.

Once I loaded some 35gr HP bullets in a 22-250 Ackley with a 1:8″ twist. After putting three shots down range, the average velocity was 4584 FPS with an RPM level of 412,560. The group measured .750″ at 100 yards. This is a clear example that it is hard to over-stabilize a good bullet.

Twist-rate illustration by Erik Dahlberg courtesy FireArmsID.com. Krieger barrel photo courtesy GS Arizona.
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July 31st, 2016

Berger Bullets Twist Rate Stability Calculator

Berger twist rate calculator

Berger twist rate calculatorBerger Twist-Rate Stability Calculator
On the Berger Bullets website you’ll find a handy Twist-Rate Stability Calculator that predicts your gyroscopic stability factor (SG) based on mulitiple variables: velocity, bullet length, bullet weight, barrel twist rate, ambient temperature, and altitude. This cool tool tells you if your chosen bullet will really stabilize in your barrel.


CLICK HERE to Go to TWIST RATE CALCULATOR PAGE

How to Use Berger’s Twist Rate Calculator
Using the Twist Rate Calculator is simple. Just enter the bullet DIAMETER (e.g. .264), bullet WEIGHT (in grains), and bullet overall LENGTH (in inches). On its website, Berger conveniently provides this info for all its bullet types. For other brands, we suggest you weigh three examples of your chosen bullet, and also measure the length on three samples. Then use the average weight and length of the three. To calculate bullet stability, simply enter your bullet data (along with observed Muzzle Velocity, outside Temperature, and Altitude) and click “Calculate SG”. Try different twist rate numbers (and recalculate) until you get an SG value of 1.4 (or higher).

Gyroscopic Stability (SG) and Twist Rate
Berger’s Twist Rate Calculator provides a predicted stability value called “SG” (for “Gyroscopic Stability”). This indicates the Gyroscopic Stability applied to the bullet by spin. This number is derived from the basic equation: SG = (rigidity of the spinning mass)/(overturning aerodynamic torque).

Berger twist rate calculator

If you have an SG under 1.0, your bullet is predicted not to stabilize. If you have between 1.0 and 1.1 SG, your bullet may or may not stabilize. If you have an SG greater than 1.1, your bullet should stabilize under optimal conditions, but stabilization might not be adequate when temperature, altitude, or other variables are less-than-optimal. That’s why Berger normally recommends at least 1.5 SG to get out of the “Marginal Stability” zone.

In his book Applied Ballistics For Long-Range Shooting, Bryan Litz (Berger Ballistician) recommends at least a 1.4 SG rating when selecting a barrel twist for a particular bullet. This gives you a safety margin for shooting under various conditions, such as higher or lower altitudes or temperatures.

Story idea from EdLongrange. We welcome reader submissions.
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October 22nd, 2015

Berger Updates Bullet BC Data and Recommended Twist Rates

Berger BC Ballistics Coefficient Barrel Twist Rate  Updates

Berger has released two important informational updates for its line-up of bullets. First, the Ballistics Coefficients (BCs) have been updated for the vast majority of bullets Berger sells. In addition, G7 model BCs are being provided for most of the bullets. You will want to use the updated BC data, which is based on actual testing of recent production lots of bullets.

Second, Berger is now providing a dual twist-rate recommendation for most bullets. Berger is now lists a “minimum” barrel twist rate as well as an “optimal” twist rate. To get maximum long-range performance from your bullets, use a barrel with the “optimal” rate of twist.

CLICK HERE for the latest Berger Quick Reference Sheets with updated BCs and new Optimal Twist Rates. Eric Stecker, Berger President says: “We have tested every lot of bullets‬ produced in the last several years to bring you these updated numbers for all of our bullets.”

Ballistic Coeffificent (BC) Updates with G7 Data
Berger notes: “We have updated all of our Ballistic Coefficients to be even more accurate.
Prior to 2008, all of Berger Bullets’ BCs were calculated using a computer prediction. Early in 2009, we began measuring BCs with live-fire testing. As a result, Berger’s BCs were updated and G7 BCs were also made available. This represented a dramatic improvement in the accuracy of performance data at that time. Since 2009, the BCs assessed for Berger Bullets have not been updated. As part of our ongoing effort to provide shooters with the best information possible, Berger has been testing every lot of bullets produced for the last several years. The result is updated and highly accurate running averages of BCs for recent production lots.

Here are some of the Updated BC Values for popular Berger Target (Match) Bullets:

Description New G1 BC New G7 BC % Change
22 Cal 80gr VLD Target 0.455 0.233 +2%
22 Cal 90gr VLD Target 0.534 0.274 -3%
6mm 95gr VLD Target 0.467 0.240 -3%
6mm 105gr VLD Target 0.517 0.265 +5%
6mm 105gr Hybrid Target 0.536 0.275 -1%
6mm 115gr VLD Target 0.563 0.289 +3%
6.5mm 130gr VLD Target 0.562 0.288 +2%
6.5mm 140gr Hybrid Target 0.607 0.311 -2%
7mm 180gr VLD Target 0.683 0.350 +4%
7mm 180gr Hybrid Target 0.680 0.349 +1%
30 Cal 155gr Hybrid Target 0.478 0.245 -1%
30 Cal 185gr Hybrid Target 0.576 0.295 +1%
30 Cal 215gr Hybrid Target 0.691 0.354 -1%

CLICK HERE for Complete Table with all bullets on Berger Website

G7 Form Factor Addition
Berger also added the G7 form factor to the Ballistics Quick Reference Sheet. The analysis of form factors can be very useful when considering a bullet’s long range performance potential. Going by BC alone can be deceptive since BC includes the weight and caliber of the bullet. Form factor indicates how much drag the bullet has, which is a very important consideration for all bullets of all calibers.

NEW Dual Twist-Rate Recommendations
Recommended twist rates for bullets are commonly listed as a single value, such as 1:12” (one rotation in 12″ of barrel travel). This may be overly simplistic. There is a big gray area of marginal stability in which bullets can fly with good accuracy, but with a reduced (i.e. sub-optimal) Ballistic Coefficient. Recognizing this reality, Berger is now listing two twist rates for each bullet it makes. The first is the minimum twist needed for good accuracy, which Berger has always recommended. The second is the new optimal twist rate, which is the twist that will stabilize the bullet to a level which achieves its full performance (BC) potential. CLICK HERE For more information.

Berger BC Ballistics Coefficient Barrel Twist Rate  Updates

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September 6th, 2015

Ballistics Tip: Understanding Bullet Stability (Twist Rate and MV)

Berger bullet SG MV twist rate stability Litz

Bryan Litz has produced an informative new video on the subject of bullet stability. The video explains how stability is related to spin rate (or RPM), and how RPM, in turn, is determined by barrel twist rate and velocity. For long-range shooting, it is important that a barrel have a fast-enough twist rate to stabilize the bullet over its entire trajectory.

Detailed Bullet Stability Article
To complement the above video, Bryan has authored a detailed article that explains the key concepts involved in bullet stabilization. Bryan explains: “Bullet stability can be quantified by the gyroscopic stability factor, SG. A bullet that is fired with inadequate spin will have an SG less than 1.0 and will tumble right out of the barrel. If you spin the bullet fast enough to achieve an SG of 1.5 or higher, it will fly point forward with accuracy and minimal drag.”


CLICK HERE to READ Full Bullet Stability Article by Bryan Litz

Berger bullet SG MV twist rate stability Litz

There is a “gray zone” of marginal stability. Bryan notes: “Bullets flying with SGs between 1.0 and 1.5 are marginally stabilized and will fly with some amount of pitching and yawing. This induces extra drag, and reduces the bullets’ effective BC. Bullets in this marginal stability condition can fly with good accuracy and precision, even though the BC is reduced. For short range applications, marginal stability isn’t really an issue. However, shooters who are interested in maximizing performance at long range will need to select a twist rate that will fully stabilize the bullet, and produce an SG of 1.5 or higher.”


Berger twist rate calculatorBerger Twist-Rate Stability Calculator
On the updated Berger Bullets website you’ll find a handy Twist-Rate Stability Calculator that predicts your gyroscopic stability factor (SG) based on mulitiple variables: velocity, bullet length, bullet weight, barrel twist rate, ambient temperature, and altitude. This very cool tool tells you if your chosen bullet will really stabilize in your barrel.

LIVE DEMO BELOW — Just enter values in the data boxes and click “Calculate SG”.

Top photo with bullet by Werner Mehl, www.kurzzeit.com, all rights reserved.
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March 4th, 2015

How Ballistic Coefficent Varies with Twist Rate (Stabilization)

By Bryan Litz, Applied Ballistics
Last month, in the Daily Bulletin, we talked about twist rate and muzzle velocity. That discussion was based on a detailed study published in Modern Advancements in Long Range Shooting.

More Spin, Less Drag
In this article, we look at how twist rate and stability affect the Ballistic Coefficient (BC) of a bullet. Again, this topic is covered in detail in the Modern Advancements book. Through our testing, we’ve learned that adequate spin-stabilization is important to achieving the best BC (and lowest drag). In other words, if you don’t spin your bullets fast enough (with sufficient twist rate), the BC of your bullets may be less than optimal. That means, in practical terms, that your bullets drop more quickly and deflect more in the wind (other factors being equal). Spin your bullets faster, and you can optimize your BC for best performance.

Any test that’s designed to study BC effects has to be carefully controlled in the sense that the variables are isolated. To this end, barrels were ordered from a single barrel smith, chambered and headspaced to the same rifle, with the only difference being the twist rate of the barrels. In this test, 3 pairs of barrels were used. In .224 caliber, 1:9” and 1:7” twist. In .243 caliber it was 1:10” and 1:8”, and in .30 caliber it was 1:12” and 1:10”. Other than the twist rates, each pair of barrels was identical in length, contour, and had similar round counts. Here is a barrel rack at the Applied Ballistics Lab:

Applied Ballistics used multiple barrels to study how twist rate affects BC.

stability gyroscopic ballistics coefficient drag twist rate

“The Modern Advancements series is basically a journal of the ongoing R&D efforts of the Applied Ballistics Laboratory. The goal of the series is to share what we’re learning about ballistics so others can benefit.” –Bryan Litz

Barrel twist rate along with velocity, atmospherics, and bullet design all combine to result in a Gyroscopic Stability Factor (SG). It’s the SG that actually correlates to BC. The testing revealed that if you get SG above 1.5, the BC may improve slightly with faster twist (higher SG), but it’s very difficult to see. However, BC drops off very quickly for SGs below 1.5. This can be seen in the figure below from Modern Advancements in Long Range Shooting.

The chart shows that when the Gyroscopic Stability Factor (SG) is above 1.5, BC is mostly constant. But if SG falls below 1.5, BC drops off dramatically.
stability gyroscopic ballistics coefficient drag twist rate

Note that the BC drops by about 3% for every 0.1 that SG falls below 1.5. The data supports a correlation coefficient of 0.87 for this relationship. That means the 3% per 0.1 unit of SG is an accurate trend, but isn’t necessarily exact for every scenario.

It’s a common assumption that if a shooter is seeing great groups and round holes, that he’s seeing the full potential BC of the bullets. These tests did not support that assumption. It’s quite common to shoot very tight groups and have round bullet holes while your BC is compromised by as much as 10% or more. This is probably the most practical and important take-away from this test.

To calculate the SG of your bullets in your rifle, visit the Berger Bullets online stability calculator. This FREE calculator will show you the SG of your bullets, as well as indicate if your BC will be compromised (and by how much) if the SG is below 1.5. With the stated twist rate of your barrel, if your selected bullet shows an SG of 1.5 (or less), the calculator will suggest alternate bullets that will fully stabilize in your rifle. This valuable online resource is based directly on live fire testing. You can use the SG Calculator for free on the web — you don’t need to download software.


Learn More About SG and BC
Barrel Twist Rate Velocity Modern Advancements Book Bryan Litz Applied BallisticsThis article is just a brief overview of the interrelated subjects of twist rate, Gyroscopic Stability, and BC. The coverage of twist rates in Modern Advancements in Long-Range Shooting is more detailed, with multiple live fire tests.

Other chapters in the book’s twist rate section include:
· Stability and Drag – Supersonic
· Stability and Drag – Transonic
· Spin Rate Decay
· Effect of Twist rate on Precision

Other sections of the book include: Modern Rifles, Scopes, and Bullets as well as Advancements in Predictive Modeling. This book is sold through the Applied Ballistics online store. Modern Advancements in Long Range Shooting is also available in eBook format in the Amazon Kindle store.

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November 12th, 2014

Zediker Writes about Long Range Shooting with the AR-15

Looking to shoot an AR-platform rifle out past 500 yards? Then you should read two recent articles by AR guru Glen Zediker. Author of The New Competitive AR-15 and The Competitive AR15 Builders Guide, Zediker is an expert when it comes to AR-platform rifles — he knows as much as any guy around. Glen believes ARs have excellent long-range capability, provided they are built to high standards, with good barrels. Glen says: “a properly configured AR-15 is easily capable of good performance at 500+ yards. Good performance means it can hit a 1-foot-square target all the time. Competitive shooters can cut that standard in nearly half (the X-Ring on an MR1 600-yard NRA High Power Rifle target is 6 inches, and high X-counts are commonplace among more skilled shooters).”

Published in the Cheaper than Dirt Shooter’s Log, Zediker’s pair of articles cover the history and upgrading of the AR-15. Part One reviews the AR’s development as an accurate firearm, tracing its evolution from a Vietnam-era combat weapon to what is now a favored target rifle of High Power competitors. READ PART ONE.

Long Range AR AR-15 Glen Zediker Cheaper than dirt

Part Two discusses the specifics that make an AR accurate at 500 yards and beyond. Zediker talks about barrel configuration (profile and twist rate), bullet selection, floating handguards, and proper mounting of optics or iron sights. READ PART TWO.

Long Rang AR AR-15 Glen Zediker Cheaper than dirt

Here are some highlights from Long-Range AR-15 Part TWO:

Barrel Twist Rate
To stabilize anything longer than a 68- or 69-grain bullet, the barrel twist rate must be — at minimum– 1-in-8. Twist rates reflect how far the bullet travels along the lands or rifling to make one complete revolution. So, 1-in-8 (or 1-8, 1:8) means “one turn in eight inches.” I think it’s better to go a little faster in twist. There is nothing wrong with a 1:7 twist. The 90-grain bullets require a 1:6.5, and that is getting on the quick side. If you want to shoot Sierra 77s or equivalent, and certainly anything longer, 1:8 is necessary. By the way, it is bullet length, not weight, which constitutes the necessary twist rate to launch a stable bullet.

Optics Mounting
Correct optical sight positioning can be a challenge. With a flattop upper, I need a good inch additional forward extension at the muzzle side of the upper for the sight mount bases to avoid holding my head “back” to get the optimal view through the scope. A longer rail piece is necessary for my builds as a result.

Buttstock Length and Adjustment
An adjustable buttstock is valuable, and even more valuable if it’s well-designed. Mostly, a standard stock is too short, and the cheek area sits too low. Adding length helps a lot by itself. There are assemblies that replace the standard buttplate to allow for length and, usually, height and rotation adjustments for the buttpad. An elevation-adjustable cheekpiece is a big help to attain a solid position.

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August 6th, 2014

Berger Updates Free Online Bullet Stability Calculator

Berger Bullets has improved its online stability calculator. Tests have shown that bullets can suffer from reduced BC if the bullet rpm (spin rate) is less than optimal, even if barrel twist rate is otherwise fast enough to stabilize bullets in flight. Now, the improved, free Stability Calculator can determine if you need a faster-twist barrel to enjoy the best BC from your bullets.

CLICK HERE for FREE Berger Twist Rate Stability Calculator

By Bryan Litz, Chief Ballistician for Berger Bullets
We’re happy to announce a major upgrade to our Twist Rate Stability Calculator which is free to use on the Berger Bullets webpage. The old stability calculator was pretty basic, and would simply return a gyroscopic stability number based on your bullet, twist rate, and atmospheric conditions. This was used to determine if your barrels twist rate was fast enough to stabilize a particular bullet or not, based on the Gyroscopic Stability Factor (SG) being greater than 1.4.

Berger Bullet Stability Calculator Twist Rate Bryan Litz

Stability and BC — How Bullet RPM Affects Ballistic Coefficients
The new calculator still calculates SG, but also goes much further. In addition to calculating stability, the upgraded calculator can also tell you if your stability level is harming the effective BC of your bullets or not. Extensive testing has proven that bullets fired with stability levels between 1.2 and 1.5 can fly with excellent precision (good groups), but suffer from a depressed BC, sometimes as much as 10%. Shooting the bullets from faster twist rate barrels allows for the bullets to fly better and realize their full BC potential.

Berger Bullet Stability Calculator Twist Rate Bryan Litz

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May 4th, 2014

Are You Spinning Your Bullets Fast Enough? Twist Rate Calculator Predicts Gyroscopic Stability

Berger twist rate calculatorBerger Twist-Rate Stability Calculator
On the Berger Bullets website you’ll find a handy Twist-Rate Stability Calculator that predicts your gyroscopic stability factor (SG) based on mulitiple variables: velocity, bullet length, bullet weight, barrel twist rate, ambient temperature, and altitude. This very cool tool tells you if your chosen bullet will really stabilize in your barrel.

LIVE DEMO BELOW — Just enter values in the data boxes and click “Calculate SG”.

How to Use Berger’s Twist Rate Calculator
Using the Twist Rate Calculater is simple. Just enter the bullet DIAMETER (e.g. .264), bullet WEIGHT (in grains), and bullet overall LENGTH (in inches). On its website, Berger conveniently provides this info for all its bullet types. For other brands, we suggest you weigh three examples of your chosen bullet, and also measure the length on three samples. Then use the average weight and length of the three. To calculate bullet stability, simply enter your bullet data (along with observed Muzzle Velocity, outside Temperature, and Altitude) and click “Calculate SG”. Try different twist rate numbers (and recalculate) until you get an SG value of 1.4 (or higher).

Gyroscopic Stability (SG) and Twist Rate
Berger’s Twist Rate Calculator provides a predicted stability value called “SG” (for “Gyroscopic Stability”). This indicates the Gyroscopic Stability applied to the bullet by spin. This number is derived from the basic equation: SG = (rigidity of the spinning mass)/(overturning aerodynamic torque).

Berger twist rate calculatorIf you have an SG under 1.0, your bullet is predicted not to stabilize. If you have between 1.0 and 1.1 SG, your bullet may or may not stabilize. If you have an SG greater than 1.1, your bullet should stabilize under optimal conditions, but stabilization might not be adequate when temperature, altitude, or other variables are less-than-optimal. That’s why Berger normally recommends at least 1.5 SG to get out of the “Marginal Stability” zone.

In his book Applied Ballistics For Long-Range Shooting, Bryan Litz (Berger Ballistician) recommends at least a 1.4 SG rating when selecting a barrel twist for a particular bullet. This gives you a safety margin for shooting under various conditions, such as higher or lower altitudes or temperatures. Try changing the altitude and temperature in the calculator and you will see that the SG can increase or decrease when these environmental factors change. Under optimal circumstances you should aim for a 1.4, that way if you change circumstances you are still over 1.1.

Erik Dahlberg rifling illustration courtesy FireArmsID.com.

Story idea from EdLongrange. We welcome reader submissions.

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February 7th, 2010

Palma Projectiles: 155s vs. the Heavier 30-Caliber Bullets

Rifle Palma bulletsIn December, we published the rules controlling choice of caliber and bullet weight for Palma competition. (CLICK HERE to Read.) In the USA, some events are still limited to 156gr or lighter bullets for .308-caliber shooters. But where such restrictions don’t exist, many shooters are using heavy 175-190gr bullets in their .308s. Is the heavier bullet always better? What considerations favor the lighter 155gr-class bullets in Palma competition? Top Palma shooter Kelly Bachand addresses these questions in today’s commentary.

Factors That Favor the 155s
by Kelly Bachand
It is clear that 155gr bullets are adequate. In the Palma game, more matches have been won and more 450 scores have been shot with 155gr bullets than with any other weight projectile. With the NRA allowing heavier bullets in Palma matches, many shooters prefer to shoot the longer, heavier bullets when possible. With their higher BCs, the longer bullets would seem to offer a ballistic advantage. There may be an edge, but in my opinion, it comes at a high cost.

Shooting a heavier bullet adds complications. You’ll probably need a different powder and new load development will have to be done. New zeroes must be calculated and confirmed on the range. An expensive new barrel with a faster twist rate may be needed to stabilize the heavier bullets. And unless you’re lucky, there’s a chance that the new barrel is less accurate than your existing barrel. (If you’ve got a “hummer” barrel for the 155s, what are the odds of getting another one as good for the 190s?) For a shooter who only competes domestically and who has plenty of time and money, this does not pose much of a problem, as he can work up multiple loads and acquire multiple barrels, or even build up a second complete rifle. But shooting BOTH the 155s and the heavy bullets (which may require a new barrel) certainly adds to the cost of competing, and the time required to work up loads. One who also competes internationally has much more to worry about, since you’ll likely be switching between the heavy bullets for most domestic matches and the 155s for most international matches.

Rifle Palma bullets

Consider the challenges you’ll encounter switching between a heavy-bullet domestic load and a 155gr international load. Will changing between two different loads (with very different recoil levels) alter your gun-handling and follow-through? Will having two loads (with different ballistics) create confusion when making wind calls? And if you DO shoot both 155s and 190s, should you have two different barrels, or should you stick to one barrel which is adequate for both bullet weights, but perhaps not optimal for either? The heavier bullets typically have a better BC which means they should be less bothered by the wind. At the same time the heavier bullets travel at a much slower velocity. Does this negate the ballistic advantage? You need to check the ballistic tables carefully, looking at BOTH BC and velocity.

As a person who prefers to keep things simple and stick with what works, it’s no surprise I continue to shoot 155gr bullets exclusively in Palma competitions. But I understand this is just one viewpoint. A Palma shooter reading this should survey competitors who are consistently putting themselves in the winner’s circle. Talk to top shooters and then make your own rational, informed decision about which bullets to use. Good luck and keep them in the center.

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April 25th, 2009

FREE Ballistics Calculators And Data Charts from Canada's NFA

Ever wondered what twist rate is required to stabilize a particular bullet? Or would you like to see how changes in spin rates (rpm) affect bullet stability? Well thanks to our friends from Canada, you’ll find helpful formulas online that answer many questions about external ballistics.

The Ballistics Page for the Canadian National Firearms Association (NFA) website offers a variety of useful programs and data charts created by Peter Cronhelm. These include:

Caliber Twist Rate Chart

Greenhill Formula Twist Calculator

Bullet Stability Calculator (requires MS Excel)

Ballistic Coefficients Chart (MS Word), (Or DOWNLOAD HERE.)

Ranging Ballistics Computer (Scroll to bottom of page.)
This exterior ballistics computer works in conjunction with range data as well as computing a conventional drop and windage chart. Using an FFP (Final Firing Point) and multiple TRP’s (Target Reference Points) the system will simultaneously calculate Drop and Windage data for up to 30 TRP’s 360 degrees around the FFP. Windage is corrected for the TRP direction compared to the primary wind direction. The spreadsheet consists of six individual pages. Each page performs a distinct function and contains all the information required to complete a shot at a target or targets. The entire system can be used in any laptop or handheld computer capable of running MS Excel or Excel CE.

CLICK HERE to download Ranging Ballistics Computer (requires MS Excel)

Rimfire Ammo Comparison Table
Canada’s NFA even provides a detailed table with bullet weights and velocities for over 100 varieties of 22LR Rimfire ammunition from Aguila, CCI, Eley, Federal, Fiocchi, Lapua, PMC, Remington, RWS, and Winchester. This Rimfire Ammo Table is a “must-have” resource for any smallbore shooter. Shown below is the section for Lapua:

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