As mentioned above, it often helps to go 1/2- 3/4 turn
past contact with the shellplate.
I didn't have a chambering issue, but I was checking headspace with a gauge for a bolt gun and wanted to bump the shoulders back .002 less than the fireformed brass. Couldn't make it happen.
Then someone suggested what I did above. There can still be some "slack" when you adjust for light contact. Adjust it so that it really cams over hard- and it'll bump the shoulders back.
The cautionary note above is legit, although I really don't think the manufacturers' tolerances will allow for enough bump to cause a headspace issue unless you've done some grinding on the die base...
Best to get a heaspace gauge, then you know for sure. Can't beat the Hornady Lock N Load:
http://www.hornady.com/store/Lock-N-...h-Body-1-Each/
And, you should
go with small base sizing dies for an AR. Guarantee it'll help with the sizing issues.