Frank Castle (Thomas Jane) uses a pair of heavily customized Colt M1911A1s as his main sidearms in the film. His father Frank Castle Sr. (Roy Scheider) devoted some dialogue to the guns, commenting on how he bored the chambers, customized the triggers, and added some thread-on compensators. Some of the other noticable feattures are their Novak night sights, their bevelled magazine wells, combat hammer and combat triggers, extended slide stop safeties and extended slide releases, as well as a beavertail grip safeties and front cocking serrations.
M4A1
Frank Castle is seen using an M4A1 fitted with an M203 grenade launcher and an aimpoint M68 red-dot sight when assaulting Howard Saint's club Saints and Sinners. He loses use of this weapon when a bullet from enemy fire hits the reciever and ruins the gun.
Special
M203 grenade launcher
Castle's M4A1 is seen fitted with an M203 grenade launcher, which do to such close proximity battles is never used during the shootout in the club.
Beretta 92FS Inox (flashpaper)
Howard Saint (John Travolta) is seen using a Beretta 92FS Inox "Flashpaper" gun, which is designed to imitate the flash and smoke of a blank gun minus the loud sound and dangerous wad which fires from the barrel. Instead of swapping to a real Beretta, it appears to stay a flash-paper gun throughout the film, so I'm seperating it from the real Inoxes used.
Beretta 92FS Inox
The stainless steel Beretta 92FS Inox makes several appearances in the film in the hands of nameless thugs. The most notable user is Dante (Marc Macaulay), one of Saint's hired thugs, who tries to draw it when facing Castle with Spoon (Terry Loughlin) at his side in the Saint building.
Heckler & Koch G36C
When Bobby Saint (James Carpinello) tries to impress his father by buying guns, he meets with Otto Kreig (Frank Castle under-cover), who shows Bobby 4 suitcases loaded with Heckler & Koch G36C assault rifles. I didn't both adding all 4 suitcases because it looks like they just reuse the same shots twice.
Colt Officers ACP
When the FBI raids the weapons transfer, Otto Kreig (Frank Castle under-cover) draws a tutoneColt Officers ACP which apears to be customized, before being "killed".
Micro Uzi
Several of Micro Uzis are seen used by Saint's men when they massacre Castle's family. Most notabley seen using them are Dante and Spoon when they both fire their Micros at Maria Castle's (Samantha Mathis) car, and manages to flip it with them.
Mossberg 500 (sawed-off)
When Saint's men massacre Castle's family, one of the hitters is armed with a sawed-off Mossberg 500 shotgun, which looks to be a smooth-bore gun based on the raises ribbing for adding a sight (which is usually used for bird guns to make swing-shooting easier.) Castle picks up this hitter's Mossberg and arrives too late to save his wife and son, finding them dead on the docks after being run over. When the people who killed them pull back around in their Ford pickup truck, Castle fires off four shots, which pattern like buckshot rounds, before running dry. When Castle first begins firing, he clearly racks a live round from the chamber, which would have greatly helped him out when he faces the men as they exit the truck. While the ribbing on the barrel is usually used for bird guns, it is not likely used as a one because bird shotguns have a 2 round cut-off in the mag tub (making a round in the chamber 3 rounds, the maximum capacity allowed for bird hunters). Since the gun is clearly seen loaded with 5 rounds, it is assumed it is used precisely as a gun to fire buckshot. (Then again, most bird hunters don't used sawed-off shotguns anyway).