I was about 18 when I did this. I bought a new, unchambered 12 gauge barrel from Numrich. Miller Lamb put a breechplug in it. I filed the breech end octagon and installed a drum and nipple. I don't remember if I installed a rib, but probably just made a thimble and soldered it onto the barrel. It shot ok, with no problems. The barrel I bought had a thick breech end as it was made to be used with many different shotguns. If the barrel is thin, you may have problems with the drum blowing out.
My best friend made a rifle using the cut-off muzzle end of a Spencer rifle barrel. It was a bit thin, so Miller made a piece that he silver soldered to the side of the barrel so the threads of the drum had more thickness to tap into. I am not sure what the good folks here think of this way to use a piece of civil war rifle barrel, but Chuck still shoots it. (Made about 1966 when we were young and poor)