I've never heard of a 12ga gas shell, but I have a break action tear gas gun in what must be about 4ga. An uncle of mine was manager of a textile mill in middle GA back when they were having union disputes, and the mill owners bought several of the guns in case of riot.
My gun looks about like the single shot grenade launchers I used in the Army back in the 60's and 70's...remember the one painted in tiger stripes the guy used in "Apocalypse Now". I also have a single shot pistol that my grandfather called a 'very-gun' he brought back from France in WW-1; it looks to be about 8ga. I'm not sure whether the very-gun was used for gas delivery or just for signal.
Your 12ga gas shell was probably for military or police use...Geo
