How many shots will a barrel last? Often asked and stated. Some years ago, I decided to come up with a way to calculate it then get answers paralleling what really happens.
My first data source was Sierra Bullets as they used 10-shot groups periodically testing them during production runs. I shot matches with their ballistics technician who hand loaded virtually all the rounds with their bullets testing them for accuracy in rail guns. Sierra used "standards;" a lot of bullets for each caliber that tested the very best for accuracy. (They used to sell 'standards' at rifle matches; they're the ones that won most of the matches and set the records.) Typically sub 2/10ths MOA for match bullets and sub 3/10ths MOA for hunting bullets. The second was winners and record setters in high power match rifle and benchrest competitors' disciplines. The criteria was how many shots before the average of few shot test groups opened up 50% over what they started at.
Sierra would periodically shoot 10 "standards" through their test barrels and track the average group size. When that barrel's average got 50% larger than the average when new, it was considered worn out for best quality control purposes. Benchrest people had the same answers. So did the high power folks. So starting accuracy to 50% bigger was their standard; at least in my opinion. But these numbers were for the most accurate barrels made held most consistently across several strings of fire. Their data usually showed a reasonably straight line of accuracy decline to that limit.
Surprising to me, everyone gave me data that when their powder charge weights in grains for a given caliber paralleled the barrel's bore cross sectional area in square millimeters, they got 3000 rounds of barrel life. Example; 22 calibers have 24 square millimeters of cross sectional area, 24 calibers have 28. 30 caliber bores have 45 and that's a typical charge weight for .308 Win. cartridges. Sierra and the high power folks rebarreled their .308 Win tack drivers at 3000 rounds.
What about cartridges that use more powder than what equals their bore area in square mm's? It just so happened that the high power folks shooting .30-06 ammo before using the .308 got about 2500 rounds of barrel life relative to my standard. 30 caliber magnums got about 1200 round. When the 7mm Rem Mag became "the" cartridge for 1000 yard matches after a good friend set the record at the Nationals with one, they got 800 rounds. And soon after the .243 Winchester hit the firing lines in high power, winner's barrels lasted about 1500 rounds. 2000 for the .260 Rem and its wildcat parent, the 6.5x.308 used in matches. In the late '90's when the 6.5x.284 became popular for long range, 800 rounds was the limit. They all closely matched what Sierra got.
I did some math comparisons and saw that when a given bore area's charge weights were increased about 40%, barrel lives were only about 50% of that 3000 round number. When doubled, it was only 25% as many.
That's when I came up with my definition of "bore capacity." That's when a cartridge charge weights for SAAMI spec average peak pressure has the same numeric value in grains as the bore area in square millimeters. Bore capacity had to be something relative to the bore, in my opinion.
Further data gathering showed e that varmint hunters' rifles got a little more barrel life; their accuracy standard (about 1/2 MOA at 100) was not as stringent and were about 30% to 50% more than competition barrels. Big game hunters even more barrel life; twice as much; 6000 rounds for bore capacity charge weights starting out with a 1 MOA average at 100 yards. And service rifles for combat accuracy at three or more times as many but they started out at about 2 MOA at a hundred yards; 9000 to 10,000 rounds is when their throat erosion gauges read "10" at the established limit for military barrel life. 30 caliber M1 and M14 match grade rifles' were rebarreled at about 5000 rounds when the erosion gauge read "5" but when at about 3000 rounds, they were often issued to new team members starting out as they didn't quite "cut the mustard" like top ranked shooters did.
Mathematically, it turned out to be about the same as the inverse square law. Square root of 2 is 1.414. If a bore's cartridges have 41.4% more powder in them, squaring 1.414 = 2. Dividing 3000 by 2 gives 1500. .243 Win cartridges burn about 41% more powder than the 6PPC and got half its barrel life; 1500 rounds compared to 3000 for the 24 caliber PPC round. .300 Wby Mag's in competition got 750 rounds burning twice the powder as the .308 Win. My .264 Win Mag. long range rifle got 640 rounds of barrel life going from pretty good to horrible over 25 shots.
Read an internet article a few years ago that Black Hills Ammo (I think) ran a test with a match rifle shooting their .308 Win. match ammo starting out at about 1/4 MOA average for 10-shot groups in their hundred yard test range. Accuracy declined at a fairly consistent rate through 8000 or more rounds to about 9/10ths MOA. Tried every search method I know of and cannot find it any more.
One thing I and others noticed; the more overbore your cartridge is, the knee of the accuracy curve drops off much faster at the end of its life. Sometimes over as few as 25 to 50 shots going from acceptable to horrible. Another recent observation; some powders burn hotter and have a higher erosion rate. Quickload software has an input for different powder's heat index; higher numbers for the more erosive ones. I was given software that calculates barrel life based on powder heat indexes as well as bore capacity that's based on about 3500 rounds of barrel life. It's results are about 20% more life than my formula; its developer may have used a larger starting out group size.
All this stuff is a relative comparison tool. Everyone has their own standards they go by.
My first data source was Sierra Bullets as they used 10-shot groups periodically testing them during production runs. I shot matches with their ballistics technician who hand loaded virtually all the rounds with their bullets testing them for accuracy in rail guns. Sierra used "standards;" a lot of bullets for each caliber that tested the very best for accuracy. (They used to sell 'standards' at rifle matches; they're the ones that won most of the matches and set the records.) Typically sub 2/10ths MOA for match bullets and sub 3/10ths MOA for hunting bullets. The second was winners and record setters in high power match rifle and benchrest competitors' disciplines. The criteria was how many shots before the average of few shot test groups opened up 50% over what they started at.
Sierra would periodically shoot 10 "standards" through their test barrels and track the average group size. When that barrel's average got 50% larger than the average when new, it was considered worn out for best quality control purposes. Benchrest people had the same answers. So did the high power folks. So starting accuracy to 50% bigger was their standard; at least in my opinion. But these numbers were for the most accurate barrels made held most consistently across several strings of fire. Their data usually showed a reasonably straight line of accuracy decline to that limit.
Surprising to me, everyone gave me data that when their powder charge weights in grains for a given caliber paralleled the barrel's bore cross sectional area in square millimeters, they got 3000 rounds of barrel life. Example; 22 calibers have 24 square millimeters of cross sectional area, 24 calibers have 28. 30 caliber bores have 45 and that's a typical charge weight for .308 Win. cartridges. Sierra and the high power folks rebarreled their .308 Win tack drivers at 3000 rounds.
What about cartridges that use more powder than what equals their bore area in square mm's? It just so happened that the high power folks shooting .30-06 ammo before using the .308 got about 2500 rounds of barrel life relative to my standard. 30 caliber magnums got about 1200 round. When the 7mm Rem Mag became "the" cartridge for 1000 yard matches after a good friend set the record at the Nationals with one, they got 800 rounds. And soon after the .243 Winchester hit the firing lines in high power, winner's barrels lasted about 1500 rounds. 2000 for the .260 Rem and its wildcat parent, the 6.5x.308 used in matches. In the late '90's when the 6.5x.284 became popular for long range, 800 rounds was the limit. They all closely matched what Sierra got.
I did some math comparisons and saw that when a given bore area's charge weights were increased about 40%, barrel lives were only about 50% of that 3000 round number. When doubled, it was only 25% as many.
That's when I came up with my definition of "bore capacity." That's when a cartridge charge weights for SAAMI spec average peak pressure has the same numeric value in grains as the bore area in square millimeters. Bore capacity had to be something relative to the bore, in my opinion.
Further data gathering showed e that varmint hunters' rifles got a little more barrel life; their accuracy standard (about 1/2 MOA at 100) was not as stringent and were about 30% to 50% more than competition barrels. Big game hunters even more barrel life; twice as much; 6000 rounds for bore capacity charge weights starting out with a 1 MOA average at 100 yards. And service rifles for combat accuracy at three or more times as many but they started out at about 2 MOA at a hundred yards; 9000 to 10,000 rounds is when their throat erosion gauges read "10" at the established limit for military barrel life. 30 caliber M1 and M14 match grade rifles' were rebarreled at about 5000 rounds when the erosion gauge read "5" but when at about 3000 rounds, they were often issued to new team members starting out as they didn't quite "cut the mustard" like top ranked shooters did.
Mathematically, it turned out to be about the same as the inverse square law. Square root of 2 is 1.414. If a bore's cartridges have 41.4% more powder in them, squaring 1.414 = 2. Dividing 3000 by 2 gives 1500. .243 Win cartridges burn about 41% more powder than the 6PPC and got half its barrel life; 1500 rounds compared to 3000 for the 24 caliber PPC round. .300 Wby Mag's in competition got 750 rounds burning twice the powder as the .308 Win. My .264 Win Mag. long range rifle got 640 rounds of barrel life going from pretty good to horrible over 25 shots.
Read an internet article a few years ago that Black Hills Ammo (I think) ran a test with a match rifle shooting their .308 Win. match ammo starting out at about 1/4 MOA average for 10-shot groups in their hundred yard test range. Accuracy declined at a fairly consistent rate through 8000 or more rounds to about 9/10ths MOA. Tried every search method I know of and cannot find it any more.
One thing I and others noticed; the more overbore your cartridge is, the knee of the accuracy curve drops off much faster at the end of its life. Sometimes over as few as 25 to 50 shots going from acceptable to horrible. Another recent observation; some powders burn hotter and have a higher erosion rate. Quickload software has an input for different powder's heat index; higher numbers for the more erosive ones. I was given software that calculates barrel life based on powder heat indexes as well as bore capacity that's based on about 3500 rounds of barrel life. It's results are about 20% more life than my formula; its developer may have used a larger starting out group size.
All this stuff is a relative comparison tool. Everyone has their own standards they go by.
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