The SIG-Sauer P226 is the weapon carried by all of the Massachusetts State Police officers in the movie, including Sergeant Colin Sullivan (Matt Damon), Detective Brown (Anthony Anderson), MSP Staff Sergeant Sean Dignam (Mark Wahlberg) and numerous other characters.
In real life, the P226 has been standard-issue for the Massachusetts State Police since 1987. However, during the scene where Collin is sitting in a class on ballistics while at the academy, the instructor mentions that the department's service pistols are chambered for 9mm hollow-point rounds. This is a slight inaccuracy; the Staties have been using the .40 S&W version of the P226 since the mid-1990s (and more recently switched to the double-action-only DAK model, whereas the guns in the movie are SA/DA models).
In some scenes, the SIG-Sauer P228 is seen carried by Sergeant Colin Sullivan (Matt Damon). A stated-as-verified screen-used SIG-Sauer P228 used by Matt Damon is currently on display at the National Firearms Museum located at the NRA Headquarters in Fairfax, VA. While the above screencaps all show what appear to be P226 pistols rather than P228's, it is possible P228s were also used as standby guns.
Although the SIG-Sauer P226 is standard-issue for the Massachusetts State Police (both in the movie and in real life), a close-up of Collin Sullivan (Matt Damon) firing his weapon at the academy reveals an Astra A-100. This pistol looks similar to the SIG-Sauer pistols, but it is a very different weapon. When Billy Costigan (Leonardo DiCaprio) is shown training on firearms at the academy, he also seems to be using this same pistol (probably the same prop gun).
Frank Costello (Jack Nicholson) carries a Beretta 84 "Cheetah" (the early model) as his sidearm throughout the movie. It is seen most notably when he confronts Billy about the possibility that he might be an informant.
Billy Costigan (Leonardo DiCaprio) carries as his pistol throughout the movie a flat nickel Walther PPK. This weapon is later used by trooper Barrigan (James Badge Dale) and then eventually by Colin.
Mr. French (Ray Winstone) carries a snub-nose version of the Colt Python throughout the film. It is seen most notably during the warehouse scene, when he cocks the weapon while it's concealed behind his back.
A Mossberg 500 Cruiser is used by Delahunt (Mark Rolston) during a shootout with Internal Investigations officers. Another Mossberg 500 Cruiser is also seen when Costello's men clear out of a room after being tipped off about a police raid.
One of the Massachusetts State Police officers is seen firing an Ithaca 37 shotgun at Costello's men during the final gunfight. It is equipped with an extended magazine tube. Costello's thugs also point Ithaca 37s at the backs of the machine-gun armed Chinese thugs.
Olympic Arms K23Bs, Canadian-made ultra-compact Colt Carbine variants with 6.5-inch barrels, are used by several Boston Police Special Operations Unit officers when they take down Costello at the end of the film.
Error creating thumbnail: File missingAn Olympic Arms K23B similar to those seen in The Departed - 5.56x45mm. The ones in the movie have A1E1-style upper receivers (forward assist and case deflector with A1 sights), whereas this gun has an A2 upper receiver and the newer-style collapsible stock, but the barrel and forearm are identical.Error creating thumbnail: File missingBoston Special Ops officers ambush Costello's men with ultra-compact Olympic Arms K23Bs.Error creating thumbnail: File missingClose-up of an officer firing his K23B. The brass deflector is just visible in this shot, as is the forward assist and A1 rear-sight, indicating an A1E1 upper (as found on most Canadian AR variants).
AKMS
An AKMS-type assault rifle is carried by one of the Chinese thugs in the warehouse (it looks like it might be a Chinese Norinco Type 56-1, but it's hard to tell because the weapon is never seen clearly).
M16A1 and M16A2 rifles are used by the police officers who perform the 3-volley salute at a funeral (all of them have A2 hand guards, but the receivers of some rifles are clearly A1 style).
During the warehouse scene, one of the Chinese thugs carries a Heckler & Koch MP5A3 fitted with a Surefire dedicated fore-end (but with no flashlight in the grip).
Two of Costello's men fires at the Massachusetts State Police with a MAC-10 at the end of the film, despite Costello's earlier statement that his men don't carry machine guns though he probably meant at business transactions.
