My "rule of thumb" was developed back in the 1970's using barrel life in 22 through 30 caliber rifles used in competition by folks shooting the best scores and groups. It's basically the number of rounds a barrel lasts starting out at some sub 1/4 MOA level at 100 yards (for benchrest rifles) or 1/2 MOA level at 600 yards (high power match/service rifles) to where it degrades 50% to 3/8ths or 3/4ths MOA. Using some emperical deductions with powder charge weights and the cross sectional area of the bore in square millimeters, I learned that a barrel would last about 3000 rounds of accurate life to that standard when 1 grain of powder was used for each square millimeter of the bore's cross sectional area. About 21 grains for 22 caliber, 27 grains for 24 caliber, 45 grains for 30 caliber.
Doubling the charge weight for a given caliber reduced barrel life to 1/4th as many rounds; 90 grains of powder in a 30 caliber barrel gave 750 rounds of barrel life base on the standard used. For charge weights inbetween, I used the inverse square laws of math to calculate them. For example, a 30 caliber magnum burning 67 grains of powder (.30-.338, .308 Norma) would get about 1125 rounds of barrel life; that's when top level competitors rebarreled their 1000-yard match rifles. And my .264 Win. Mag. long range match rifle barrel lasted 640 rounds. Folks shooting the 7mm Rem. Mag. in long range matches in the '70's got about 800 rounds of barrel life.
Sierra Bullets rebarreled their rail guns' .308 Win. match grade barrels after about 3000 rounds of barrel life testing most of their 30 caliber bullets 180 grains and lighter. Their .300 Win. Mag barrels testing heavier 30 caliber bullets got rebarreled at about 1000 rounds.
For other types of rifles, the life numbers got bigger. A factory sporting rifle starting out at 1/2 MOA at 100 yards may show twice the barrel life; about 6000 rounds for a .308 Win, .222 Rem or about 2300 rounds for a .308 Norma Mag. M14 and M16 service rifles as well as the M1 had barrel lifes 3 times as long; 9000 rounds or more based on a 1 to 2 MOA accuracy level at 100 yards. Same for sporting rifles starting out at that level.
At one extreme of the barrel life issue is smallbore .22 rimfire match rifles. They used to get about 50,000 rounds of 1/2 MOA accuracy at 100 yards until the priming mixtures were changed in the 1980's. Since then, they last about 30,000 rounds of 3/4 MOA at 100 yards. Most of the smallbore records shot before then still stand. Smallbore match ammo ain't what it used to be...
That spreadsheet gives number bigger/larger than what my rule of thumb is based on. Such is life when emperical formulation's used to get an educated guess for an answer. But the important thing to see is the more powder burned in a given caliber, the shorter the barrel life will be. To say nothing of the errors introduced into accuracy assesment by the shooter's own lack of tack-driving marksmanship skills; something that has to be accounted for if reality is the objective.
There is some disagreement on rapid fire versus slow fire rates effecting barrel live across the barrel makers. Some believe that normal or reduced loads shot rapid fire don't change my numbers but extra hot long range loads will shorten it. And some powders seem to erode barrel steel more than others. My numbers were mostly attained from stainless steel barrels but a lot of the ones for 30 caliber rifles came from chrome moly match grade service rifle barrels; there was not a significant difference. And there will be other variables to some degree that impact the numbers derived by actual shooting tests and accuracy standards as well as component selection and assembly.
I just wanted to get some "ruler" based on meaningful numbers that was better than personal opinions based on all sorts of egos, marksmanship levels and expectations. To that end, I think Mikecr did a pretty good job of putting some good icing on my decades old barrel life cake.
