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Rifles, Reloading, Optics, Equipment
Reloading
Measuring to the lands
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<blockquote data-quote="Bart B" data-source="post: 1122688" data-attributes="member: 5302"><p>When a .270's fired, the bullet jump distance to the rifling's controlled by the distance from the case shoulder to the bullet's contact diameter on its ogive somewhere between a diameter of .277" down to .270". The case head is not against the bolt face but a few to several thousandths away from it. There's a few thousandths spread in case headspace; distance from case head to shoulder that's easily measured. Depending on the chamber's actual headspace, there could be several thousandths slop between the case and chamber lengthwise. Therefore, measuring a loaded round's bullet rifling contact diameter to the case head is a waste of time ; at least in my opinion.</p><p></p><p>Several thousandths spread in bullet jump to the rifling is not a big concern anyway. It'll increase .001" for every few dozen shots fired anyway using the same bullet seating depth or cartridge overall length. Barrel steel erodes away from high pressure hot gases when fired. Match grade AR10, M1 and M14 rifles shooting the same lot off match ammo over 3000 to 4000 rounds of super accurate barrel life have their last bullets jump near 1/10th inch further to the lands than when the barrel was new. Same with bolt action rifles shooting ammo loaded from their box magazines with cartridge OAL the same across their barrel life.</p><p></p><p>If you can shoot your stuff into no worse than 1/10th MOA at 100 yards, you may see a 1/20 MOA loss of accuracy as the rifling erodes 1/10th inch further down the barrel increasing bullet jump that much. You could seat bullets .001" shallower in the cases every few dozen rounds to compensate for it..</p></blockquote><p></p>
[QUOTE="Bart B, post: 1122688, member: 5302"] When a .270's fired, the bullet jump distance to the rifling's controlled by the distance from the case shoulder to the bullet's contact diameter on its ogive somewhere between a diameter of .277" down to .270". The case head is not against the bolt face but a few to several thousandths away from it. There's a few thousandths spread in case headspace; distance from case head to shoulder that's easily measured. Depending on the chamber's actual headspace, there could be several thousandths slop between the case and chamber lengthwise. Therefore, measuring a loaded round's bullet rifling contact diameter to the case head is a waste of time ; at least in my opinion. Several thousandths spread in bullet jump to the rifling is not a big concern anyway. It'll increase .001" for every few dozen shots fired anyway using the same bullet seating depth or cartridge overall length. Barrel steel erodes away from high pressure hot gases when fired. Match grade AR10, M1 and M14 rifles shooting the same lot off match ammo over 3000 to 4000 rounds of super accurate barrel life have their last bullets jump near 1/10th inch further to the lands than when the barrel was new. Same with bolt action rifles shooting ammo loaded from their box magazines with cartridge OAL the same across their barrel life. If you can shoot your stuff into no worse than 1/10th MOA at 100 yards, you may see a 1/20 MOA loss of accuracy as the rifling erodes 1/10th inch further down the barrel increasing bullet jump that much. You could seat bullets .001" shallower in the cases every few dozen rounds to compensate for it.. [/QUOTE]
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Rifles, Reloading, Optics, Equipment
Reloading
Measuring to the lands
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