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 US Navy & Palma Rifle Team Member
NRA High Power Master & Long Range High Master
NRA Smallbore Prone Master