Bore Capacity versus Barrel Life

[Bart B]

How is bore capacity calculated?

I researched barrel life top ranked competitive shooters and Sierra Bullets testing bullets for accuracy got from several cartridges loaded to SAAMI spec max average pressure. Barrel life standard was when the average 100-yard test group was about 1/4 MOA then opened up to about 3/8 MOA. That's when these folks decided the barrel was worn out for their objectives.

Bore capacity had all sorts of definitions. I wanted one that was fixed for a given bore diameter; not groove diameter. That would keep overbore and underbore capacity relative to some standard. In comparing barrel lives across several cartridges, one thing stood out above all others. Bores with a cross sectional area in square millimeters had a barrel life of 3000 rounds when the powder charge weight in grains was the same number as square millimeters. 46 was the grains of powder in .308 Win cartridges that gave 3000 rounds of barrel life with the bore cross sectional area of 46 square millimeters. Other calibers fared much the same. Open the following image in a separate window.

Cartridges with about 40% more powder for a given bore had about 1/2 or 1500 rounds for the barrel life. Twice as much powder produce about 1/4 the barrel life; 750 rounds. Yellow shaded areas are questionable; there is evidence that the bore erosion rate is much faster in this area which reduces barrel life a lot more such as the 7mm STW.

These numbers were based on average powder burning temperatures. Forum member mikecr used Quickload's powder heat indexes for his formula so that characteristic would be figured in. He also used a greater number for the barrel life standard in stead of my 3000 rounds. He may have used a different accuracy degradation figure instead of 50%.

Hunting rifles may well have twice the barrel life number; 6000 or so and have bigger starting out groups. It depends on lots of things. But for service rifles like the M1, M14 and M16, their barrel life standards are about 10,000 rounds before accuracy had degraded too much for combat use with much larger groups to start out with.

This is an approximation and guideline. Individual tests may yield different results.

Bore cross sectional area in square millimeters:

* bore diameter in inches squared times .7854 times 645.

* bore diameter in millimeters squared times .7854.

 

[alcesgigas]

Is there any way that chart/image can be made larger and clear enough to read?

 

[Bart B]

Try this for image size:

 

[300RUM LNGRNGE]

Does bullet weight not play a part in this as well?

 

[DocDoc]

How does this fit in with Gunworks and their 6.5 using H1000 and supposedly getting over 2000 in barrel life. The Sherman and SS, seem to be reporting the same longer life than shown by this chart. What is different?

 

[Bart B]

Does bullet weight not play a part in this as well?

I don't think so. There's not enough change in charge weight across bullet weights to make a significant difference. Heavier charges with lighter bullets spend less time pushing them out than lighter charges with heavier bullets.

 

[Bart B]

How does this fit in with Gunworks and their 6.5 using H1000 and supposedly getting over 2000 in barrel life. The Sherman and SS, seem to be reporting the same longer life than shown by this chart. What is different?

I've no idea. Don't know the objectives, conditions and standards for their tests. Except that 3-shot test groups for their accuracy guarantee is a joke. Why limit tests to 3 shots; are their barrels fit poorly to receivers that they start stringing shots in some direction as they heat up? There's usually a 3X or more spread across several 3-shot groups.

 

[Timber338]

... Except that 3-shot test groups for their accuracy guarantee is a joke. Why limit tests to 3 shots; are their barrels fit poorly to receivers that they start stringing shots in some direction as they heat up? There's usually a 3X or more spread across several 3-shot groups.

this site is filled with interesting viewpoints and opinions, and this post is one of the most unique I've read in a while. Well done!

 

[Trickymissfit]

Ackley wrote some similar things many years back, and a couple others wrote about the issue long before he did. The data Bart supplied came about much later than Parker's, and is probably a little better. (never going to take much of anything away from Parker Ackley).

With the above aside, we must realize that we are also dealing with a generic case length and shape. Case shape has a lot to do with case life just like core capacity and even burn rates. A fine example is the long favorite .243 Winchester compared to the 6mm Remington. The Remington case will always have a longer barrel life due to the shape of the shoulder and neck length. When one checks the turbulence point (I prefer the vortex of the flame point), the T.P. of the .243 ends up being just beyond the lip of the neck. While the Remington puts it inside the neck. The data fails to show this. Now I'm not a fan boy of the 40 degree shoulder as it creates it's own set of issues. I like the 30 degree or even the 35 degree shoulder much better. Yet I'd prefer the standard .243 case with a 30 degree shoulder with a .350" neck length (HLS?) Still if I could have my own design, it would be a stretched 6BR that would have the shoulder length of something like a .250 Savage or the .243 Winchester. I think we'd see a 3000 shot barrel life.
gary

 

[Mikecr]

Rules of thumb for this includes more than powder and bore capacity. There is also load pressure, powder heat potential, and shot rate.
But by far the most difficult factor lacks any credible standard of measure: ACCURATE barrel life. This is where the numbers, even completely truthful numbers, really fall apart.

 

[Rich Coyle]

tag

 

[Bart B]

But by far the most difficult factor lacks any credible standard of measure: ACCURATE barrel life. This is where the numbers, even completely truthful numbers, really fall apart.

Good point.

How does one separate the human variables that effect accuracy from that of the rifle and ammo?

 

[alcesgigas]

Thanks Bart for the rapid response in clarifying the chart and an excellent thread.

 

[coyotezapper]

Try this for image size:

Ok, answer me this. 6.5 RSAUM running 61.5 grs H1000 59K still 1/2 moa after 3000 rounds or how about factory 7MM STW that ran hot loads 88 grs H1000 65-70K for the first 1000 rounds approximately and then another almost 500 rounds of 60-65K before the barrel puked. 6.5 has Bartlein bbl and STW a factory cryoed bbl.