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
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.”
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At SHOT Show we checked out a versatile and well-designed new tool from Forster Products — the new Datum Dial Ammunition Measurement System. This handy device measures a variety of important dimensions: 1) cartridge headspace from base of case to shoulder datum; 2) cartridge length from base of case to bullet ogive; 3) bullet base to bullet ogive; and 4) bullet bearing surface (requires two tools).
The complete Datum Dial Kit consists of a dial-holder mount and three precision machined dials. The mount clamps to your sliding-jaw calipers. The mount holds a black ring or one of two supplied gold rings. The black ring has 5 precision-diameter holes that let you check headspace (on shoulder datum) for nearly all modern cartridge types.
Swap in one of two available gold rings to measure to bullet ogive index points. The two gold dials (five holes each) give you 10 different bullet diameter choices, covering the vast majority of common pistol and rifle bullet sizes. This lets you measure both base of bullet to ogive and base of cartridge case to bullet ogive. We really like this dual functionality, as it allows you to quickly sort bullets, plus you can measure your loaded rounds as they come out of the seating die. I have found that accuracy is enhanced when you maintain close base-to-ogive tolerances on your loaded rounds.
Watch Video to See Datum Dial Features and Functions
Replace a Bin Full of Comparator Inserts
It is important to understand that the Datum Dial system replaces a comparator body and a complete set of caliber-specific comparator inserts. You no longer need a box full of inserts, with one for each caliber that you load. The Datum Dial Complete Kit includes: One dial-holder mount (which clamps on your calipers), one black headspace dial, two (2) gold bullet diameter dials, and a plastic storage box. The Datum Dial Kit is available right now. MSRP is $157.00, and “street price” should be $20 or so lower. Additional dials or caliper jaw mounts are sold separately.
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Prices of bullets and brass have gone up dramatically in recent months. We are hearing from active shooters that cost considerations are influencing their decisions about what calibers and chamberings to shoot. There is a definite trend to smaller cartridges and lighter bullets.
One match shooter told us: “I’ve been debating between a 6.5×47 Lapua and a 6-6.5×47. After comparing the cost of 6.5mm vs. 6mm bullets, I decided on the 6mm. If I save $7 bucks a box, and shoot 4000 rounds a year (40 boxes of bullets), that’s $280.00 in savings–enough to buy a new barrel.”
Here are some comparative bullet prices for 6mm, 6.5mm, 7mm, and 30-caliber bullets at Midsouth Shooters Supply. Prices are for a 100-count box. Note that the 6.5mm match bullets cost 25% more than the 6mms. For active shooters, the price difference adds up quickly. (Prices from current catalog listings; particular items may be out of stock.)
Brand
6mm
6.5mm
7mm
.308
Berger
105gr VLD $30.15
140gr VLD $37.87
180gr VLD $44.12
190gr VLD $45.37
Sierra
107gr MK $26.25
142gr MK $32.45
175gr MK $31.08
200gr MK $33.73
Here are brass costs for Lapua brass from Grafs.com. Prices are for 100-count boxes (or four 25-count boxes for the .338 Lapua Magnum). Generally speaking, the bigger the case, the higher the price (except for the .308 Win).
.223 Rem
.243 Win
6.5×47 Lapua
6.5-284
.308 Win
.338 Lapua Mag
$58.99
$93.99
$104.99
$118.99
$69.99
$264.99
Consider Barrel Life Also
Certainly, moving to a smaller caliber can often reduce what you have to pay for brass and bullets. On the other hand, you need to consider barrel life. Hot-loaded 6mms, such as a .243 Ackley, can burn up a barrel much more quickly than a .308 Winchester. In comparing the “operating costs” of various cartridges, you need to factor in barrel replacement costs as well as component prices. If you have to spend $550 (including smithing) to replace a custom 6mm barrel every 1500 rounds, you’re spending $1100 more than a guy who has a .308 Win which lasts 4500 rounds.
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Having a quality funnel helps you get carefully-measured powder charges in your cases, with no spillage, every time. If you’ve tried cheap, plastic funnels you know that they can rock on the case mouth. Also, if the fit isn’t good, some kernels end up on your bench, instead of inside the case.
Thankfully, precision, caliber-specific metal funnels are available to reloaders for about $10-$12.00. Satern Custom Machining funnels feature an aluminum top with a caliber-specific, machined brass base/neck. The Satern funnel’s aluminum body is completely static-free so powder flows into the case without bridging. Dick Saunders makes a similar, turned-aluminum funnel sized for particular calibers. Both these metal funnels make reloading easier (and they are ideal for blackpowder loading because they do not give off static electricity). As Forum member Wes notes, with a fitted metal funnel: “the possibility of the funnel sliding to the side, and spilling powder all over the place is drastically reduced.”
Satern Aluminum/Brass Funnels
Steve Satern crafts rugged aluminum funnels with brass ends. The caliber-specific sizes offer a snug fit that keeps the funnel tight on the case neck while the powder is flowing. Forum member Danny Reever reports: “If you are tired of the generic ‘fits all’ funnels falling over and powder spraying every where, try one of the Satern funnels. Sinclair Int’l sells them and they are top notch.” SMike308 adds: “I have retired my plastic funnels after buying my first Satern funnel. I now have one for each different caliber that I load for. I especially like that the Satern has the brass weight at the bottom, which adds stability to the funnel.”
Satern funnels are sold by MidwayUSA (6mm $11.29), Satern Machining (6mm $11.25), and Sinclair International (Item 11-9XX, 6mm $11.95). Satern Machining also offers two types of universal funnels, 22-30 caliber and 30-50 caliber. Satern’s universal funnels, along with the larger .338 to .50 caliber funnels, cost $18.45.
Saunders Aluminum Funnels
Some of our Forum members prefer the turned aluminum funnels made by Dick Saunders. You’ll need to order these direct from Saunders. Specify .17, .20, .22, 6mm, 6.5mm, .30 caliber or “all-purpose”. The Saunders funnels start at about $10.00. Contact Dick Saunders at the address below:
Dick Saunders
145 Delphi Rd.
Manchester, IA 52057
563-927-4026
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.223 WSSM and 6mmBR Disaster — Report by Dr. Jim Clary
Under most circumstances, shooters don’t have to worry about chambering the wrong cartridge into the wrong rifle. After all, the cartridges are well marked and we all know which rifle we are shooting on any given day. In many cases, incorrect cartridges cannot be chambered — larger cases will not fit in smaller chambers, for example. No problem! That being said, I can tell you that even an experienced, careful and normally safe shooter can make a mistake.
The following is an account of just such a mistake that could have resulted in death or dismemberment. Fortunately, the shooter was not hurt, but the rifle was completely destroyed.
Last year, a friend purchased a Savage Precision right bolt, left port, single shot bolt action in 6mmBR Norma. It was an incredible prairie dog gun and he spent the summer burning powder and busting dogs. In October, he purchased a stainless steel Browning A-Bolt Varmint in .223 WSSM. The weather in the upper Midwest turned sour by the time he got the brass tuned up and he only got to fire it a few times before he was “socked in” for the winter. Thus, he spent his evenings loading ammo for the spring thaw.
During a break in the weather, he grabbed both rifles and a couple of bags of .223 WSSM and 6mmBR cartridges and headed to the range to check out his new loads. In case you are not familiar, the 6mmBR is smaller in diameter and a mite shorter than the .223 WSSM. Because of this, it will chamber in a .223 WSSM, but the .24 caliber bullet is too big for the .22 caliber bore. That is what happened to my friend.
The rest is history; when he squeezed the trigger, all hell broke loose. The entire bottom of the rifle blew out, including the magazine assembly. The explosion actually cut the stock into two pieces. However, the bolt held and amazing as it may seem, the .243 bullet was “swaged” right out of the .223 barrel.
6mmBR (left) and .223 WSSM (right) cartridges above the remains of Browning A-Bolt rifle.
One Small Mistake Is All It Takes
Now, realize that my friend has been shooting all manner of firearms, safely, for over half a century. He is meticulous, thorough and conscientious in his approach to reloading and shooting. However, he made one mistake. He put some lose 6BR cartridges in a baggie as he packed up from a prairie dog hunt last summer, without noticing that the baggie was marked .223 WSSM in black marker. Then, when the break in his winter weather came, he grabbed the bag, believing it to be the WSSM cartridges and didn’t check the head stamp.
Couldn’t happen to you? How many times have we emptied our pockets of cartridges and dropped them into a plastic container on the shooting bench? How many times have we set down to a marathon reloading session, loading several calibers in a row? How many times have we put the wrong bullets, cases or primers into the incorrect container? My point is that even the safest of us can make a mistake. So, look at the picture above and take a bit more time when you reload your ammunition at home or chamber a round in the field. It might save your life.
COMMENT: In a thread inspired by the above story, the moderator of another gun forum wrote:
“There was a rifle (or what was left of it) mounted above the door of a range I used to go to. The story behind it was of a guy who was shooting a .30-06 and set it aside to shoot his .25-06 instead. He didn’t bother putting the larger cartridges away first and of course one found its way into his gun. The explosion took three fingers off his left hand, two off his right and stuck a piece of the bolt in his face. He recovered but was never the same again. The pieces of the gun were gathered and mounted to show others why it pays to be mindful of what you’re doing. It was effective as there was never another such accident at that range.”
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The complete Datum Dial Kit consists of a dial-holder mount and three precision machined dials. The mount clamps to your sliding-jaw calipers. The mount holds a black ring or one of two supplied gold rings. The black ring has 5 precision-diameter holes that let you check headspace (on shoulder datum) for nearly all modern cartridge types.
Having a quality funnel helps you get carefully-measured powder charges in your cases, with no spillage, every time. If you’ve tried cheap, plastic funnels you know that they can rock on the case mouth. Also, if the fit isn’t good, some kernels end up on your bench, instead of inside the case.
