WWWJD, some years ago a friend and I each shot one .308 Win. case a lot of times to see how many full length sizings it would take. I got 47 times and he got 57; that's when we both ran out of test powder. Both of used maximum loads. The dies were standard .308 Win. RCBS ones that I had lapped their necks out to about 2 thousandths smaller than a loaded round's neck diameter. We both used metered charges of IMR4895 under Sierra bullets; he used 168's and I used 165's.
His ammo was fired from a machine rested rifle at 100 yards; all 57 rounds went into 3/10ths inch center to center. My ammo was shot by a friend from benchtop holding it to his shoulder; all 47 went into 3/4ths inch through a chronograph showing a muzzle velocity spread of 30 fps or thereabouts.
In both tests, the case shortened about 2 thousandths when fired then lengthened a bit more than 2 thousandths. We trimmed our case about every 10 shots back to 2.000".
US Navy Distinguished Marksman Badge 153
Former USA Palma Team Member
NRA High Power Long Range High Master
NRA Smallbore Prone Master