tobnpr, yes, how much case necks are annealed is critical. But I don't think the number of rounds a case has been fired makes much difference. Not after a friend shot the same .308 Win. case 57 times putting Sierra 168's into 3/10ths MOA at 100 yards. Full length sized every time, too, then he ran out of test powder. Max loads, too, not reduced charges.
The optimal amount of neck tension's what gives the lowest muzzle velocity spread. Best way I know of to do that is to have case neck tension as small as possible so bullets don't get displaced in normal handling. Seat them long enough so they're set back several thousandths (10 to 15's good) by the rifling when the bolt's closed on them. That way, as the leade erodes away from firing, all bullets fired in one session will meet the same resistance starting into the rifling.
PA-joe and tobnpr, there are formal tools to measure neck tension. Military ammo specs call for a release force range or minimum. For example, 7.62 NATO M80 ammo has a spec of at least 60 pounds of force to get the bullet out. M118 and M852 match ammo's spec'd to at least 40 pounds.
One can easily make their own test setup. Put a collet type bullet puller in a board clamped to a benchtop, then wire a bucket onto a shell holder that slips onto the rim of a round with its bullet held by the collet. Start dumping stuff into the bucket until the bullet comes out; Heavy bullets are great if you've got several pounds of 'em. Weigh the bucket and its contents. Write down the weight in pounds. Most folks who've done this are agast at the range of weights needed to get their bullets out of a case. Also the average weight to do so. Especially when compared to factory and military ammo.
US Navy Distinguished Marksman Badge 153
Former US Navy & Palma Rifle Team Member
NRA High Power Master & Long Range High Master
NRA Smallbore Prone Master
Last edited by Bart B.; January 7, 2013 at 10:31 PM.