Unlike the original Lethal Weapon (where a Beretta 92F was carried), Detective Martin Riggs (Mel Gibson) carries a Beretta 92FS as his primary sidearm in this film, and as such, uses it frequently throughout the film. In a memorable scene, Riggs empties the magazine of his Beretta into one of the main villain's henchmen while remembering the names of his friends and loved ones. Other LAPD detectives are also seen with Berettas.
Smith & Wesson Model 19
The Smith & Wesson Model 19 appears as the primary weapon of Detective Roger Murtaugh (Danny Glover). In his signature Lethal Weapon move, Murtaugh can be seen rolling his neck before taking a shot.
During the raid of the South African residence, Detective Collins (Grand L. Bush) can be seen wielding a Beretta 92SB-C. Not a 92FS Centurion or a 92FS Compact, as they weren't released until 1992, and since this film was released in 1989, it would have to be a 92SB Compact Version. It is also noted as a 92SB-C by its rounded trigger guard.
Heckler & Koch MP5K
The Heckler & Koch MP5K appears in the film as one of the weapons used by the main villain's henchmen, first seen in the South African embassy being carried under a coat via a shoulder sling, then again aboard the cargo vessel Alba Varden during the final battle.
Heckler & Koch MP5A3
The Heckler & Koch MP5A3 appears in the film as another weapon used by the main villain's henchmen, first during the raid on Riggs' beachfront mobile home (seen with and without brass catchers), then again during the final battle aboard the Alba Varden, two of which are commandeered and used by Riggs and Murtaugh.
Mauser C96 Bolo
During the final battle aboard the Alba Varden, Arjen Rudd (Joss Ackland) fires on Riggs with a Mauser C96 before running out of ammunition.
Walther PPK
Rudd's chief henchman Pieter Vorstedt (Derrick O'Connor) uses a Walther PPK, sometimes fitted with a silencer.
A Walther P5 is used by one of Rudd's henchmen as he commandeers a tow truck.
SIG-Sauer P228
A suppressed SIG-Sauer P228 pistol is used by one of Rudd's henchmen, when he tries to murder Leo Getz (Joe Pesci) in the hotel room while posing as room service. [Edit} I don't think it's a P228. On the close up you can see that it doesn't have the trigger guard of a P228, I think it's either a P226 or a P220.
I agree. Also possibly a P225, since the size is similiar to the P228 and the P229 wasn't released yet.
Ithaca 37
An LAPD officer searches a building with an Ithaca 37 shotgun.