When Charlie came up with the idea to sell merchandise at the bar, Frank came up with the idea of selling a "Gun Shot" - which is a gun that shoots liquor into your mouth. The first version was a real gun with a hooker throwing tequila in someone's face. Later, Frank made tequila bullets with real gunpowder.
When Frank shows Dennis and Mac his idea for the Gun Shot, they changed it from a handgun to a shotgun and improved it. They also decided to make a commercial for it rather than develop a working product. In the commercial that they pitch to a room of confused and concerned investors Mac and Dennis use a Ithaca Model 37 Featherlite 12 gauge shotgun that has presumably had certain internals removed. ("Paddy's Pub: Home of the Original Kitten Mittens")