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 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.”
John Barsness published the same information in Rifle 216, April 2002, pp26-33, “Efficient Cartridges”. I guess great minds (or Johns) think alike.
Comment by D Pepper — May 5, 2008 @ 12 pm
Whoops!! That was “Handloader” not “Rifle”…sorry.
Comment by D Pepper — May 5, 2008 @ 12 pm
Interesting. Is there any actual data (not folklore) to support this?
Also – why use bore area? The amount of barrel steel to erode or wear out is proportional to bore perimeter (or diameter), not to area.
Comment by Josh — May 5, 2008 @ 1 pm
I agree with Josh. Volume/area is a poor proxy for “overboreness”. I haven’t thought it through yet but since a .50 bmg is basically a scaled up ‘06 it should equal a 30′06 in “overboreness”. It would serve us better to examine different methods that would reflect this.
Editor: There may be better, or more sophisticated formulas we can develop. Looking at the Index Chart, I think John’s approach has merit for initial comparative purposes. It’s not the “final answer”, but it’s a good start in my mind.
Quote: “Since a .50 BMG is basically a scaled up ‘06 it should equal a 30-06 in ‘overboreness’.”
NOT True at all….
When comparing the 30-06 vs. 50 BMG, remember that the volume of a cylinder goes up with the SQUARE of the Radius of the column: pi*r2*h
This means that though the 50 BMG is only 1.406″ longer, i.e. 56% longer, than a 30-06, it actually has VASTLY larger case capacity. The 30-06 has 68.20 grains of capacity, while the 50 BMG has nearly 300 grains! 300 grains is 440% of the 30-06 capacity! That’s why the 50 BMG can and should be considered much more overbore than the 30-06. The 50 BMG may look like a 30-06 clone but the ratio of length to Volume is WAY different.
30-06: 2.494″ case length, 68.2 grains capacity
50 BMG: 3.900″ case length, 293-300 grains capacity
DATA from Quickload
Comment by Ninth Stage — May 5, 2008 @ 10 pm
I think the weight of the bullet you are trying to fire also has bearing to if a case is overbore. A .243 firung a 60gr bullet is way overbore, but the same 243 firing a 115HPBT is less so. Over-bore is a term for efficiency, if you are using more powder than actually needed to attain a certain velocity. You can shoot the same 60gr 6mm bullet out of a PPC with the saem speed as with a 243win, but the latter requires a lot more (slower) powder.
Comment by Dasher — May 6, 2008 @ 11 am
Editor – I’ve posted on my site a better description of what I meant and a chart of the same cartridges calculated for overbore by my method.
- Ninth
Comment by Ninth Stage — May 6, 2008 @ 3 pm
WOW! Nice discussion going here. It seems pretty clear that John L. was attempting to define ‘overbore’ as an index of barrel life alone, while Dasher and Ninth Stage would add something like ‘being too large to make effective use of available powders’. So thanks to all of you for a nice exchange.
John L. – how about addressing your use of bore cross-sectional area rather than bore surface area?
Editor – I’m glad you defended the material you published, but maybe you better read the post on Ninth Stage’s site.
Ninth Stage – nice job.
Comment by Josh — May 6, 2008 @ 6 pm
In John L’s table, there is an incorrect value for the 270 Win bore diameter, it is 0.277, not 0.266. This changes the 270 Win index to 1095.2, placing it between the 22-250 and 257 ackley.
Also, a volume passing through an area can be equated to velocity if the time for the expansion is considered. A higher gas velocity, i.e., more burnt powder through a smaller bore, results in higher erosion rates. I think John L’s index is meaningful in this regard.
—–
Editor: Thanks for noting the error. It has been corrected.
Comment by Rex — May 6, 2008 @ 10 pm
The index does use the cross section area of the bore as suggested by Josh.
Rex, your comment is correct, all I was trying to come up with is an expression that mathematically compared different cartridges and that attempts to quantify the old “overbore” description.
The part I find really interesting is that the cartridges used most in benchrest and other forms where accuracy is the key have the ratio’s much lower than many cartridges.
John
Comment by John L — May 7, 2008 @ 11 am
Sometimes it helps to test a theory with extreme cases to see if it still makes sense. Let’s examine John’s with some fictional extreme cartridges. These cartridges all have bore diameter cylindrical interiors 6″ long. Let’s try these cartridges with bores of .308, .600, and 36″ (yeah, that 36″ one makes a .45 ACP pale in the fat and stubby department).
The .308 has a water capacity of 112.8 grains (to make it easy to do your own calculations, a cubic inch of water weighs 252o .306 grains). The .600’s capacity is 428.1 grains and the 36″ tub holds a whopping 1,540,875 grains of water. Plugging these numbers into John calculations we get the number 1514 for all three. That’s right, all three cartridges are as “overbore” as the other.
John’s equations calculate the grains per square inch. In fact, you can work that number I gave you above, less rounding errors, by dividing 1514 by 6.
BTW, by my method, the .308 gets an index of 4915.5, the .600 gets 2523.5, and 42.1 for the 36″ monster.
Comment by Ninth Stage — May 7, 2008 @ 12 pm
I apologize. When you said, “There may be better, or more sophisticated formulas we can develop”, I thought that a meaningful discussion might then take place. I see now that I was wrong and that it is time to “be quiet” as Josh said. I will continue to develop my thoughts on “overboreness” by myself.
I’d hoped that we could do this together. John’s and my charts are not so far apart, but you seem married to one of them.
Comment by Ninth Stage — May 8, 2008 @ 11 pm
Two issues for me – the barrel throat is being worn out initially by the gas volume/temperature of the powder charge while the effects of friction from the bullet traveling down the barrel is wearing out the tube. Most barrels go “bad” from chamber / throat area erosion not the tube being worn out. However that said the higher the velocity and surface area of the bullet the more the tube is worn so it is a factor. Powder charge vs the bore diameter is important to me as a predictor of potential wear in the first few inches of the barrel. Simply stated less powder or larger bore equals less wear.
Comment by Salty — May 10, 2008 @ 4 pm