Things to check
1- Headspace with that brass. If the rims are too thick, it may affect lockup.
2- locking lugs fully engaged?(check with a felt tip pen). If not, the interlock will partially block the hammer fall. Install heavy lug spring from Bellm
3- Weak hammer spring
4-Make sure the stock bolt is not going into the frame too far(add a washer).
Firing pin free?
