nut
Sir:
I assume you know it's a nut since you see you need a wrench - those were put in with a nut.
I'd take the bbl out, clean it up and check the threads and align it and put some loc-tite on the threads and with an improvised wrench screw the nut in pretty tite, but not stripping the threads. Then put a piece of flat lead or soft metal and smack the end of the bbl. and see if the nut will go further.
Ought to hold.
Harry B.