The SIG-Sauer P226 is carried by all of Massachusetts State Police (MSP) officers in the film, including Sergeant Colin Sullivan (Matt Damon), Detective Brown (Anthony Anderson), Staff Sergeant Sean Dignam (Mark Wahlberg) and numerous other characters.
Note: In real life, the P226 has been standard-issue for the MSP members since 1987. However, during the scene where Colin 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 MSP 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 film are SA/DA models).
In some scenes, the SIG-Sauer P228 is carried by Sergeant Colin Sullivan (Matt Damon). While the above screencaps all show what appear to be P226 pistols rather than P228s, it is possible P228s were also used as standby guns.
Note: 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, Virginia.
Although the SIG-Sauer P226 is standard-issue for the Massachusetts State Police (both in the film and in real life), but a close-up of Colin 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 seen training on firearms at the academy, he also seems to be using this same pistol (probably the same prop gun).
Billy Costigan (Leonardo DiCaprio) carries a flat nickel Walther PPK as his sidearm throughout the film. Later this weapon is used by Trooper Barrigan (James Badge Dale) and then eventually by Colin Sullivan (Matt Damon).
Frank Costello (Jack Nicholson) carries a Beretta 84 Cheetah (the early model) as his sidearm throughout the film. It is seen most notably when he confronts Billy about the possibility that Billy might be an informant.
Arnold French (Ray Winstone) carries a Colt Python (snub nose model) throughout the film. It is seen most notably during the warehouse scene, when he cocks the weapon while it's concealed behind his back.
During the warehouse scene, one of Chinese thugs carries a Heckler & Koch MP5A3 fitted with a Surefire dedicated forend (but with no flashlight in the grip).
Error creating thumbnail: File missingHeckler & Koch MP5A3 with Surefire 628 dedicated forend weaponlight - 9x19mmError creating thumbnail: File missingA Chinese thug holds his Heckler & Koch MP5A3 with a Surefire grip (for some reason, there is no flashlight or any other attachment mounted in the grip).
Cobray M11/9
Two of Chinese thugs are seen carrying Cobray M11/9 submachine guns during the money exchange in the warehouse.
Near the end of the film, two of Costello's men are seen firing MAC-10 submachine guns at the Massachusetts State Police, despite Costello's earlier statement that his men don't carry machine guns.
A Mossberg 500 Cruiser is used by Timothy Delahunt (Mark Rolston) during a shootout with Internal Investigations detectives. 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 Massachusetts State Police officers is seen firing an Ithaca 37 shotgun at Costello's men during the final shootout. It is equipped with an extended magazine tube. Costello's thugs also point their Ithaca 37s at the backs of 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's thugs near 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 film 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 their ultra-compact Olympic Arms K23B carbines.Error creating thumbnail: File missingA close-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 Chinese thugs in the warehouse. It looks like 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 handguards, but the receivers of some rifles are clearly A1 style.
