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
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.”
Similar Posts:
- “Overbore” — A Practical Definition by the Numbers
- Defining “Overbore” Cartridges — The Great Debate
- Defining “Overbore” Cartridges via Comparative Index
- How Long Will Your Barrel Last? Dan Lilja Offers Some Guidelines
- How Long Will Barrels Last? Dan Lilja Lists Factors to Consider
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Tags: .243 Win, Ammunition, barrel life, Cartridge, Overbore, Overbore Index
I have to admit, I find myself surprised by some of the rankings. The .270 higher than the 22-250? The 25-06 higher than the .220 Swift? I may need to reconsider some of my preconceptions. Thanks for putting together the chart.
It seems this subject requires further exploration and explanation.
Case capacity has a direct relationship to the powders required in terms of burning rate, flame temperature and duration. Probably more important is the abrasive residual carbon to grind away at the throat after each shot.
Bullet weight, style and length selection are a function of a barrel’s twist rate and all contribute to powder selection for most all those cartridges on the scale nominated as overbore.
Norma ran tests to see whether bullet coatings had a significant influence on barrel wear with some amazing results.
A similar test for different powder burn rates would also be very interesting.
Extremely expensive and very time consuming, but interesting.
Wouldn’t mind betting that pops a few eyeballs open.
Since Bryan Litz is a well regarded ballistician and has written two books on this subject matter he might be a good source to put this issue in perspective?? What say you Mr. Litz?
It’s only overbore if you don’t have a powder for it. The term “overbore” is overused. Really no such thing.
A good initial test of a new hypothesis is “Does it make sense?”. This one is suspect. The units of the overbore index given are volume/area which equals length, so overbore is measured in inches (?), which seems to fail the common sense test.
Another reader mentions powder choice. While your 308 is overbore with Bullseye, it isn’t with 4831, so something is missing there.
There has been a better index of “overbore” readily available for some time. It’s usually called “Ballistic Efficiency”, and it is the ratio of chemical energy stored in the powder charge to kinetic energy imparted to the bullet. The Quickload software reviewed on this website calculates it as part of the results.
A few examples of Ballistic Efficiency, using load data from Hodgdons’ website and results from Quickload show quite a difference with the table above:
222 Rem, 50 gr bullet, 20.5 grs 4198, efficiency = 29.9%
50 BMG, 750 gr bullet, 250 grs US869, efficiency = 26.6%
25-06, 117 gr bullet, 60.5 grs Retumbo, efficiency = 22.6%
Josh is onto it here.
Efficiency is the issue. Case capacity “overbore” is irrelevant if you don’t include the volume of the barrel, hugely influenced by length, not just bore diameter.
Quickload isn’t a perfect load guide, but there isn’t anything better available to hand loaders.
A relative efficiency guide would be interesting, but if you have Quickload, you already have one.
If the point of the exercise is to determine an index that roughly correlates to barrel life then I think the answer to the common sense question has to be yes! If the information is applied and compared to real-world results, I would anecdotally agree that it is surprisingly accurate, however there are numerous contributory factors. What is missing of course is a compendium of evidence to back it up, and therefore this must be viewed as a guideline. Of course I can stand up and tell you that your 6.5-284 or 7 SAUM will no longer be reliably competitive past 1000 rounds, and someone will pipe up and say theirs is just getting broken in with that many rounds.