O (Takashi Sorimachi) uses two Beretta 92FS in Macao, a standard and a customized one with a compensator and extended magazine. One of the target's bodyguards also uses one. Police officers are seen armed with these as well. When escaping from his hideout in Singapore, he uses another one with wooden grips.
In the opening scene, O uses a Browning Hi-Power to kill his target. Later on, Lok Tok-Wah (Andy Lau) picks up a nickel plated one for the final shootout with O.
Numerous variants of the Remington 870 are seen throughout the film. Lok Tok-Wah (Andy Lau) uses a sawed-off one in his opening scene at a police station hidden in a bouquet of flowers and then another one in downtown Hong Kong. O uses a full-size 870 with a vertical foregrip and a custom stock when escaping from his hideout.
O has a Franchi SPAS-12 in his arsenal and a police officer uses one when clearing a building. Both instances lack stocks, meaning they could perhaps be the same weapon reused.
Lok Tok-Wah tries to teach Chin how to shoot and trains her with an M16A2 or similar AR-15-style rifle fitted with a suppressor, scope, bipod, and 20-round magazine.
In O's real hideout, a custom AR-15 in a sniper's configuration with scope, bipod and special pistol grip is found in the arsenal. The hand guard appeals to be the Bakelite variant that is seen in an early AR-15 design. Chin uses it during their escape.
Lok Tok-Wah uses what at first glance appears to be a Barrett M82 to cover O and Chin as they escape from the police. However, on closer inspection, it actually appears to be an SKS rifle encased inside a Barrett M82 shell.
