Bart Bobbitt came up with a formula for barrel life, back in 1992 and posted it on rec.guns usenet before www gun forums existed. You can still access it through google groups... even though your ISP has probably dropped usenet.
https://groups.google.com/forum/#!topic/rec.guns/iC-CoKQCeW8
The trouble is, he was competitive at Camp Perry, and his standards were so high, his take off barrels would seem like new barrels to me.
Bart Bobbitt
11/4/92
I finally completed my research on a way to calculate how many rounds
a rifle barrel can be expected to deliver its accuracy level. By that,
I mean the barrel can be expected to have an average group size for
so many rounds before that average group size starts to get larger.
First, the rule-of-thumb formula I derived will produce a barrel accuracy
life of about 3000 rounds.
Second, if a lot of rapid fire (one shot every 5 to 10 seconds) is done,
the accuracy life will be less.
Third, if full-auto or very fast fire (a few shots per second, or one shot
every second) is done, accuracy life will be much less.
Fourth, the accuracy levels are for ranges through 600 yards. Once the
barrel life calculated limit is reached, groups will probably start to
get bigger at the longer ranges before they are noticeably bigger at the
shorter ranges.
My formula, or rule-of-thumb process, is:
1. Calculate the bore area in square millimeters.
2. Use one grain of powder for each square millimeter. This is what
I call the reference, or base powder charge.
Example: .30 caliber bore = 45.6 square millimeters.
Base powder charge for .30 caliber is 45.6 grains.
A .30 cal. cartridge that burns 45.6 grains of powder should give a
barrel life of about 3000 rounds of good accuracy.
3. If a larger cartridge is used and it burns more powder, the
accuracy life in rounds for that bore size is reduced. The amount
of reduction is determined by
a. Divide the increased charge by the base charge, then square
the answer.
b. Divide that answer into 3000.
Example: .28 caliber bore has a base charge of 38.5 grains.
Cartridge burns 57.8 grains of powder.
(57.8/38.5) squared is 2.25.
3000 divided by 2.25 is 1,333 rounds.
If anyone can shoot a hole through this theory, I welcome that shot. This
is more or less an emperical process based on accurate barrel life in
several calibers as reported to me by lots of folks. All I did was study
the data and determine what math would give a best-fit formula.
And if someone has a better method, I'd like to know what it is. My
formulas may not be the best.
