FTF's (gun related) [after break end] Look at:
#1 Ramp
#2 Chamber Opening ( a lot of jambs are at the top of the opening [magazine influence combined with also])
#3 Rough breech face
#4 Extractor clearance (while your at it, look at the hook profile)
#5 Spring balanced with load
#6 slide velocity ( part of gun balancing)
Then it gets down to quality, metal burs, individual parts, etc.
Why wait until after break in?