[Volucris]

Sounds just like you're not seating the bullet correctly and the bullet is being pushed into the lands upon chambering. You're essentially pulling the bullet out of the barrel grooves and not just the chamber. Check your OAL and adjust your seater accordingly.

OR you're not resizing the brass properly and need to adjust your resizing die. A small base die is an unnecessary "fix" proposed by people who haven't figured out how to adjust their sizing dies properly. There is no need for a small base sizing die for commercial AR15s. They generally have rather loose chambers and FL sizing works 100% if done right.