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Old September 20, 2013, 01:41 PM   #9
Bart B.
Senior Member
Join Date: February 15, 2009
Posts: 6,318
cosnyka, you're right about the barrel whipping versus accuracy. It was proved a hundred years ago that bullets leaving as the muzzle axis whips up to its highest angle are compensated for the muzzle velocity they leave at. Slower ones leave later and at a higher departure angle; faster ones sooner at a lower departure angle. At the longer ranges, accuracy is better than at shorter ones. The Brit's SMLE .303's were/are famous with this with big muzzle velocity spreads using cordite. Even the military's M14 rifles demonstrated this as their gas port being mid point in the barrel bowed its muzzle angle downward a bit while the bullet moved forward in the front half of the bore. Front locking bolt action rifles typically don't exhibit this except at the very short ranges where it has a small beneficial effect.

The main reason full length sized cases tend to be more accurate than neck only sized ones is there's less interference between a case with moderate clearance between its body and the chamber than those with minimal clearance. As no chamber nor cartridge case is perfectly round, with minimal clearance there will be interference between case and chamber repositioning the round a tiny bit. That causes different waves of vibration with different amounts and directions in the barrel when the round fires. With enough clearance, that won't happen and the barrel whips the same for each shot fired.

This is the reason benchresters, for the most part, switched over to full length sizing their fired cases. But they don't reduce their fired case body diameters nor set shoulders back more than about .001"; enough to ensure clearance and no interference whatsoever for every round fired. This also ensures there will be no binding at all as the bolt's closed on a live round. Even the slightest binding of a microscopically too large of case degrades accuracy; the bolt must close to exactly the same place for every shot fired.
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