Bobby (Mark Wahlberg) uses a pistol gripped Remington 870 throughout much of the movie, and once used by Jack (Garrett Hedlund) as the shooters escape in their car.
Mossberg 500 Persuader
Bobby (Mark Wahlberg) finds a pistol gripped Mossberg 500 in the shooters' apartment, it was used to kill his mother in the hit. Bobby uses this shotgun specifically in the Mercer home shootout scene to kill multiple thugs.
This is an 870, likely the same gun from above. -GM
Heckler & Koch G36C
A couple of Victor Sweet's hoodlums use Heckler & Koch G36Cs in an attempted hit on the Mercer brothers in their mother's home, one of them fitted with a G36E-esque carry handle/sight assembly.
Heckler & Koch MP5A5
One of the SWAT officers at Fowler' house is armed with a H&K MP5A5.
Heckler & Koch MP5KA4
At least one of Victor Sweet's hoodlums use this sub-machine gun in an attempted hit on the Mercer brothers in their mother's home.
Heckler & Koch HK33
A SWAT team member uses one of these rifles on a bi-pod to shoot and kill a crazed Detective Fowler (Josh Charles). It appears to be fitted with the 30 round magazine that was introduced with the HK93 civilian rifle.
Steyr AUG
At least one of Victor Sweet's hoodlums use a Steyr AUG in an attempted hit on the Mercer brothers in their mother's home.
Colt MK IV Series 70
This is Bobby's (Mark Wahlberg) pistol which he hands off to Angel (Tyrese Gibson) from the trunk of his car during a scene of trying to get information about their mother's killers. Bobby later uses it to shoot at Damian's dogs and then at Damian as he escapes. Bobby also uses the pistol to execute one of his mother's killers after the car chase scene.
Ruger P90
The character, Damian (Lyriq Bent) retrieves a Ruger P90 from his apartment when the Mercer brothers come banging on his door. He then fires a few shots at them when attempting to escape. Angel (Tyrese Gibson) then uses this pistol throughout the rest of the movie, specifically during the car chase scene.
Magnum Research Inch. Mark VII Desert Eagle
One of Victor Sweet's thugs produces a ported Desert Eagle and fires a shot into Jack's shoulder, wounding him. Bobby (Mark Wahlberg) then picks up this same gun off the ground after the gunfight and shoots another thug pointblank after getting information out of him.
When Bobby kills the van driver, the Desert Eagle he uses changes to a "Non Gun" Desert Eagle.
Beretta 92FS
A couple of the pistols Bobby (Mark Wahlberg) ransacks from the shooters' apartment appear to be Beretta 92FS pistols.
Heckler & Koch Mark 23 ("SOCOM")
When Bobby lifts the Mattress to reveal the shooter's arsenal, a Heckler & Koch Mark 23 can be seen.
Uknown Large Frame Revolver
When Bobby lifts the mattress to reveal the shooter's arsenal, a large frame revolver of unknown make or model can be seen.
looks like a S&W Model 66, a stainless Model 19. -GM
Uknown Small Frame Revolver
When Bobby lifts the mattress to reveal the shooter's arsenal, a pair of small frame snub nose revolvers can be seen of unknown make or model can be seen.
I think it is the Colt DS seen later in the film. -GM
Beretta 92FS Inox
One of Victor Sweet's shooters (Awaovieyi Agie) reponsible for their Mother's death fires a Beretta 92FS Inox at Bobby's car during the car chase scene. A Beretta Inox is also used by Angel during the home shootout.
SIG-Sauer P220 Sport
One of the weapon that Bobby takes from the shooter's apartment is a SIG-Sauer P220 Sport which Angel later uses in the home shootout, and one of Victor Sweet's men can be see with one just before the meeting.
Colt Detective Special
Detective Fowler (Josh Charles) uses this snub nose .38 Special revolver to shoot at kill his partner Lt. Green (Terrence Howard) , Fowler then wipes off his finerprints and ditches the revolver.
Glock 17
Detective Fowler (Josh Charles) uses a Glock 17 as his issued sidearm and draws it when theres a knock at the door in one scene. Angel (Tyrese Gibson) obtains it from Fowler and holds him at gunpoint, Fowler then retrieves the pistol again and holds Angel hostage, then fires shots at the Detriot police before being shot.
What appears to be a "Non Gun" Glock can be seen in Fowler's hand after he is shot by a sniper.
Smith & Wesson Model 19
A Smith & Wesson Model 19 is seen amongst the various guns Bobby (Mark Wahlberg) ransacks from the shooters home, he aims it at Jerry when he arrives at the Mercer home suddenly unannounced while he and Jack sort through the ransacked guns.
Colt AR-15 Sporter
At Lt. Green's (Terrence Howard) funeral, police officers can be seen firing Colt AR-15 Sporters aka SP1 rifles with heavy A2 style barrels with A2 Style hand guard, and 20 round magazines into the air.
Remington Model 700PSS
Just before the police go to Fowler's house, a SWAT officer can be seen with a Remington Model 700PSS slung over his shoulder.