What do the locking lugs lock into?
I just can't remember the word. "Chamber" is not what I'm looking for, neither is "barrel extension".
The only words I can think of are "rampart", "parapet", and "embezzlement"... Which are obviously not what I need. Perhaps the word is similar to one of those, though.
This has been bugging me all day... It's a word I knew at one time, but I never use it and can't think of it now.