I always headspace on the brass I am going to use. I generally buy brass of the same lot in bunches of 500 for varmint ctgs. & BP Competition ctgs. and 200's for other rifle rounds.

When I buy a reamer for a ctg., I measure brand new brass of the same lot and have to reamer ground to .001" over brass measure - head diameter & shoulder diameter and length rim to shoulder/neck. I want as little expansion as possible - more accurate and brass lasts forever. I NEVER push a shoulder back while sizing. They've never become tight or refused to chamber and I am not a 'mild' loader. For example, my 22" 9.3x62 gets 2,519fps with 286gr. Normas and 2,675fps with 270's. Brass has over 15 firings now and is still going strong. The barrel's getting a bit long throated but still holding 1 1/4".

With belted mags, trouble looms due to loose manufacturing tolerances on headspace - best to measure all brass and divide into groups. Pick the group with the largest # of cases, usually 1/2 way, short to long (1/2 way is about .005" slop) and use that measure for the headspace - throwing out or turning back those that are too long, .001" to .005" OVER the middle of the road. this slop is why lots of buys have trouble with longevity with belted mag. cases. Any touching of the shoulders while sizing just magnifies the headspace slop. Belted cases are not needed today, except on straight cases.

Here's a suggestion for chambering a belted mag, like a .338, etc - have the reamer mfg'r grind off the belt section, then turn them off the brass cases. Result is a perfect .338 WinMag semi-rim that headspaces on the shoulder where it should. With a claw, the rim can also be turned down to head diameter and will work perfectly - true rimless .338 Mag, or 7mm mag, or whatever mag.