Woodbutcher, I have made recievers out of cold rolled steel [shown] then browned, Bronze, Aluminum and the Seven Ga. is all Stainless.
The slug shooter have faster twist rifleing to stabilize the longer bullets. The 50s are 1in22 and 1in36. Round ball shooters are 1in66 or70.
As far as brands
32 Douglas
45,54 Green Mtn
50,62,72 Orion
50,54 De Hass
7ga. Seamless stainless pipe .......proof tested with a double load and ball
The breech plug extends back into the reciever and is tighted by a set screw.