
The shotgun depicted here is a Flintlock Musket, which was originally made for the Colonial sets for Helmet Heroes. I had to thicken the gun just so I can get it to a good enough size to not look clunky.
This logic should also apply to other two-handed guns such as Winchesters, AK-47s, FN P90s (no, Robby, they're not called ES C90s), and Steyr AUGs. Maybe not heavier guns like RPG-7s and bazookas, though.
But yeah, there ya go.