Jerry (Joseph Cortese) pulls a Mauser Model 1934 and attaches a suppressor when confronting Bernie in his office. Bernie's bodyguard (William Metzo) also carries one in his pocket.
Astra Constable
Marley (Keith Davis), the hitman that eventually finds Serge and Joe, uses a Astra Constable throughout the film, often suppressed.
Serge (Bobby Alto) carries a M1911A1 with white grips, which Joe (Joe Pesci) chides him for having, claiming it can "go through six people."
O/U shotgun/rifle
The hitman Marley (Keith Davis) gets a "custom" 18-inch barrel O/U long gun that fires a shotgun shell and a rifle bullet, using it during his confrontation with Jerry. The seller/manufacturer mentions in the scene that he made some modifications (although considering the low budget of the film, it's doubtful that any were actually made.
double-barreled shotgun
Jerry uses a double-barreled side-by-side shotgun during his final confrontation with Marley.
M2 Carbine
One of the hitmen during the finale uses a selective fire M2 Carbine with extended 30-round magazine.