A Glock 23 is the sidearm carried by Boston PD Detective Thomas Craven (Mel Gibson) throughout the film. He uses it on multiple occasions, most notably to dispatch some of Northmoor's private security personnel working for Jack Bennet (Danny Huston). When Craven enters David Burnham's apartment, the "23" slide markings are clearly visible. The Glock 23 is also the real life issue weapon of Boston Police Department's officers and detectives.
Darius Jedburgh's (Ray Winstone) sidearm throughout the film is a Heckler & Koch P2000SK. He uses it a few times, most notably at the senator's house.
SIG-Sauer P220R
Northmoor's private security officers, including Jack Bennett's bodyguards Agent One (Frank Grillo) and Agent Two (Peter Epstein) all carry SIG-Sauer P220R pistols as their sidearms. Craven (Mel Gibson) also takes one from Bennett's driver and uses it to scare him.
Smith & Wesson Model 19 Snub Nose
Jack Bennett (Danny Huston) uses what appears to be a Smith & Wesson Model 19 Snub Nose during the climactic firefight. This is the only frame in which the revolver is visible and it's very fast so the screencap quality suffers somewhat.
Glock 17
A security guard working for the senator is briefly seen firing a Glock 17.
Sawed Off 12 Gauge Double Barreled Shotgun
A sawed-off 12 Gauge Double Barreled Shotgun is used by Agent One (Frank Grillo) to murder Tom Craven's daughter, Emma Craven, at the beginning of the film. One is also seen on a dead hitman's table, who's being used as a fall guy.
M4A1
The Boston PD S.W.A.T. uses M4A1 Carbines fitted with EOTech holosights and tactical lights, while serving an arrest warrant on a suspect.