Jerry (Joseph Cortese) pulls a Mauser Model 1934 and attaches a suppressor when confronting Bernie in his office. Bernie's bodyguard 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.