Here's the formula:
Calculate the bore cross sectional area by squaring the bore diameter (Bd) in millimeters then multiply the answer by .7854.
Example: .308 Win. Bore in mm = 7.62. 7.62 x 7.62 = 58.1. 58.1 x .7854 = 45.6. (.7854 is a constant you can use instead of the radius squared times pi.)
45.6 is the area of the bore and the number of grains of powder right at bore capacity. And rounds with powder charges in grains equal to bore capacity in square millimeters get about 3000 rounds of accurate life when accuracy is sub 1/4 MOA at 100 yards.
For different cartridges using more or less powder, do this:
Divide the new cartridge powder charge weight in grains by bore capacity, then square that number. Then divide 3000 by it.
Example. 70 grains of powder divided by 45.6 equals about 1.54. 1.54 squared equals 2.37. 3000 divided by 2.37 equals 1278 rounds of sub 1/4 MOA accuracy at 100 yards.
If you use the groove diameter, answers will be a tiny bit bigger, but not enough to make significant errors. Resultant answers are typically within 10% of reality. Note that with less accurate barrels, the life goes up as I mentioned earlier.
Here's the formula: 3000/((Cw/Bc)^2)
Cw is the charge weight in grains, Bc is bore capacity in square millimeters representing grains of powder.
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