Yeah check the neck diameter. If you're not sure of your chamber's neck dimension, candle soot or sharpie marker on the case neck will show you if it's touching all the way around. I use mil 7.62 brass, and definitely need to neck trim although that's for 6.5 Creedmoor.
Sometimes the necks retain a little flare at the mouth after sizing down, check for that too.
Also, check the base to shoulder datum with a comparator, compared to your fired rounds in that chamber. It's possible you may need to set the die a little deeper when forming brass than you need when just sizing fired brass; the shoulder might not be pushed back far enough in the forming operation, if it needs to be pushed back.
Last edited by Yondering; 01/04/17.
