Tipsy, I've no rule of thumb to correlate jump and increase of powder charge to maintain velocity before pressure increases unsafely as I get closer to the lands.
My only experience with such stuff at longer ranges was shooting the same 7.62 NATO match or .308 Win. commercial match loads across four Garand barrels where cartridge OAL stayed the same and the throat advanced several hundredths of an inch over its accurate life. Bullet drop differences at 600 and 1000 yards across that much throat erosion was not enough to mandate a significant elevation zero change to compensate for velocity changes. At 600 yards, a 150 fps velocity difference means almost a 3 MOA change in elevation zero; at 1000, it's a 6 MOA change. I never observed that much change in zero over the life of the barrels as their throats advanced. Perhaps 1 MOA, at most.
Meanwhile, back to throat erosion measurements. It's been a long time standard for 30 caliber Garand and M14 barrels that their throat erosion gauge advances 1/10th inch further into the origin of the rifling for every 1000 rounds of barrel life. The gauge head's .3000" diameter at its tip and an inch back, it's .3100" diameter. That's .0010" diameter change for every 1/10th inch of length. After 3000 rounds in the 7.62 NATO Garand barrels I've worn out, the gauge read 3 numbers higher than it did when new; accuracy was degraded enough to not be good for top level competition. At 5000 rounds, it read 5 higher; the limit for lower level competitions and that's when the military shops supporting match grade rifles rebarreled them. "10" was the reading limit for regular service rifles at about 10,000 rounds.
Due to the gauge's taper differing than that of bullets ogive's .308" diameter down to .300", that's why the gauge will go in further than bullets for a given dimensional change at the origin of the rifling.
My only experience with such stuff at longer ranges was shooting the same 7.62 NATO match or .308 Win. commercial match loads across four Garand barrels where cartridge OAL stayed the same and the throat advanced several hundredths of an inch over its accurate life. Bullet drop differences at 600 and 1000 yards across that much throat erosion was not enough to mandate a significant elevation zero change to compensate for velocity changes. At 600 yards, a 150 fps velocity difference means almost a 3 MOA change in elevation zero; at 1000, it's a 6 MOA change. I never observed that much change in zero over the life of the barrels as their throats advanced. Perhaps 1 MOA, at most.
Meanwhile, back to throat erosion measurements. It's been a long time standard for 30 caliber Garand and M14 barrels that their throat erosion gauge advances 1/10th inch further into the origin of the rifling for every 1000 rounds of barrel life. The gauge head's .3000" diameter at its tip and an inch back, it's .3100" diameter. That's .0010" diameter change for every 1/10th inch of length. After 3000 rounds in the 7.62 NATO Garand barrels I've worn out, the gauge read 3 numbers higher than it did when new; accuracy was degraded enough to not be good for top level competition. At 5000 rounds, it read 5 higher; the limit for lower level competitions and that's when the military shops supporting match grade rifles rebarreled them. "10" was the reading limit for regular service rifles at about 10,000 rounds.
Due to the gauge's taper differing than that of bullets ogive's .308" diameter down to .300", that's why the gauge will go in further than bullets for a given dimensional change at the origin of the rifling.
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