## Defining “Overbore” Cartridges — The Great Debate

*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.”

### Similar Posts:

- What is an “Overbore” Cartridge? Let’s Look at Some Numbers…
- Defining “Overbore” Cartridges via Comparative Index
- “Overbore” — A Practical Definition by the Numbers
- How Long Will Your Barrel Last? Dan Lilja Offers Some Guidelines
- Can You Predict Useful Barrel Life? Insights from Dan Lilja

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Tags: Caliber, Cartridge, Case, Gunsmithing, Overbore, Overbore Index

You also have to keep everything in perspective also. Whilst the .264 Win Mag is waaaay overbore a hunter chasing speed goats seasonally with one is not likely to every wear the barrel out (hunting accuracy) in his life time. Horses for courses.

It’s hard to quibble over the relative placements of each cartridge produced by the formula. But I think it’s actually a simpler concept than the ratio. The way accuracy degrades is that the lands wear down. Only the lands very near the throat do the engraving into the bullet jacket, and they wear off much more rapidly than further down. To the extend the grooves open up too, it’s also near the throat. If the bullets were press fitted slowly through barrels, lands would last multiples of times longer. Just like hammering a bullet against a steel plate only deforms the bullet, but shooting the plate deforms both the plate and bullet. So, it’s the pressure behind the bullet times the length of the bearing surface against the first part of the lands – that is really what’s wearing the steel down. How many grains of water the case holds doesn’t account for short or long bullets passing into the lands.

Enter the 50 BMG, as merely a scaled up 30-06, it is a curiosity. Both cartridges operate at similar psi and their muzzle velocity is similar. The 50 weighs 4 times as much with 4 about times the actual powder charge. The chart shows very different barrel wear rates. From the point of view of the leading edge of the lands, the 50 is not pushing any harder than the 30.06. But the 50 is a far longer bullet so that front engraving part of each of the lands that is doing all the work, does so on a much longer jacket. Steel is equally hard in both barrels, so under the same psi, only the length of copper being displaced really differs.

Something That I find interesting in this article is that no mention of pressures.

For example from what i have read the .260 remington at 60k is considered to be more of a barrel burner over bore cartridge than the 6.5 swede at 51k. It makes me wonder if the formula needs to be tweeked to include pressure. I am not an expert and may be wrong.