At the beginning of the film, the Baby's Mother (Ramona Pringle) draws a Walther PPK from her purse and uses it to try and kill the 1st Killer (Wiley M. Pickett). The gun jams on her but luckily Mr. Smith (Clive Owen) is there just in time to save her. After killing 1st Killer with a carrot (We know it's insane), he fixes the malfunctioned gun and uses it for the entire first shoot out before running out of ammo.
Magnum Research Desert Eagle Mark XIX
Mr. Karl Hertz (Paul Giamatti) fucking uses a satin nickel Magnum Research Desert Eagle Mark XIX chambered in .50 AE (you can tell this, because the Mark XIX .357 and .44 Magnum models all have fluted barrels, this one does not) and fitted with a custom muzzle break as his sidearm in the film, which the director chose as his weapon because he improperly believed it to be "the most powerful handgun in the world" and wanted to portray Hertz as a "pussy with a gun in his hand", as quoted in the film.
A notable mistake in the film is during the scene in which Hertz interrogates Donna Quintano (Monica Bellucci) or "D.Q" by burning her with the hot barrel of his Desert Eagle, firing it more times increasingly to make it hotter. Smith interrupts and Hertz points his Desert Eagle at him, only for Smith to tell him he's "blown his load", claiming the gun is a six-shooter, when in fact the .50 AE model holds seven shots plus one in the chamber. For that matter, none of the Desert Eagle variants hold six shots. It can be assumed that the writer of the script intended Hertz to have a magnum revolver, such as the Smith & Wesson Model 500, instead of the Desert Eagle.
Taurus PT92 AFS
Smith (Clive Owen) takes a Taurus PT92 AFS off of one of the thugs on the rooftop and uses it for a good 1/4th of the film.
Glock 17 pistols (both 2nd and 3rd generation) are used by several of Hertz's (Paul Giamatti) thugs throughout the film. At one point in the film, Smith takes a 3rd gen Glock 17 from a dead thug and holds it on Hertz, but Hert tells him the gun has the same thumb print safety as his Desert Eagle (which conveniently fits right in the thumb relief). Smith then reveals the severed hand of the owner and activates the safety just in time to shoot Hertz in the bullet proof vest before he stabs him with a hunk of glass. He then remarks, "Nothing like a good hand-job." We later see him produce a tutone model with a chrome slide in the Hammerson factory, which he tosses away, setting off a trip wire and killing a thug holding a gun on him. This same tutone Glock is used by the Diner Holdup Leader (David Ury) at the end of the film.
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Beretta 92FS
Beretta 92FS pistols are used by many of Hertz's thugs. Smith (Clive Owen) uses one during the skydiving shoot out.
Smith (Clive Owen) is seen holding a Beretta 93R on an early promotional poster for the film.
It appears to be a "Fake" 93R, mocked up from a 92FS. The giveaway is the slide-mounted safety, barely visible.
SIG-Sauer P226
During the shootout in his house, Smith grabs a tutone SIG-Sauer P226 with a newer milled slide and uses it throughout the shootout before it runs dry. Smith also uses a K-Kote model taken from a secret service agent to hold up Senator Rutledge (Daniel Pilon) on his plane and uses it during the ridiculous sky diving shootout. This is strange because the secret service should be using P229s.
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SIG-Sauer P226R
SIG-Sauer P226R pistols (SIG P226s with picattiny rails on the frame nose) are used by several of Hertz's men. Technically these are now the official P226s but on this site we still call them P226Rs for easier categorization.
SIG-Sauer P228
Aside from the SIG P226 Smith draws off a Secret Serviceman, the other USSS members use SIG-Sauer P228 pistols, despite the fact that the USSS currently uses P229s, not P228s.
Heckler & Koch USP-9
Smith (Clive Owen) takes a Heckler & Koch USP-9 with a stainless slide off of a thug in his house after his SIG runs out of ammo. He mainly uses it for the shootout in the Hammerson factory.
Smith is prominantly seen wielding two of these tutone USPs on the film cover, although for whatver reason they have the slide of a Desert Eagle digitally edited onto them.
Heckler & Koch USP Expert
A tutone Hechler & Koch USP Expert is used by the Diner Hood with Earing (Mike Rad) before Smith (Clive Owen) kicks it from his hand. Because all his fingers were broken and casted, he catches the gun on a carrot held firmly in the cast and uses the carrot as a finger while hitting the gun with his other hand to fire it.
Smith & Wesson Model 629
The Lone Man (Greg Bryk) uses a stainless steel Smith & Wesson Model 629 with a 6" barrel and Hogue grips as his weapon of choice in the film. According to the director, The Lone Man's attachment to the .44 Magnum is apparently sexual, as he often "cleans" it in the bathroom. Byrk was somewhat embarrassed when he was informed of this fact, unaware of this character trait when he signed on for the role.
Para-Ordnance Nite-Tac
Para-Ordnance Nite-Tac pistols in both stainless and black are used by Hertz's men when Hertz (Paul Giamatti) teams up with Hammerson (Stephen McHattie), whom the guns are produced from. The logo by Para-Ordnance is shaved off and Hammerson's logo is added instead with laser engraving and backfilling.
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Para-Ordnance Nite-Hawg
Mr. Bill Hertz (Paul Giamatti) uses a Para-Ordnance Nite-Hawg as his "spare rod" or back up gun. Smith (Clive Owen) takes this pistol from him at the end of the film and uses it during the final showdown.
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Heckler & Koch P7
Heckler & Koch P7 pistols (not able to confirm them being an M8s or M13s) are seen in the pawn shop when Smith (Clive Owen) goes to buy bullets. It can be assumed that these pistols are CO2 copies or airsoft replicas, as are all the guns in the shop.
Browning BDM
A Browning BDM is seen on a pistol rack in the pawn shop. Since it closely resembles the BDM but has some different traits, it is likely a cheap airsoft imitation.
Smith & Wesson 4506
A pair of Smith & Wesson 4506s are seen in the pistol case in the pawn shop. Based on looks, they appear to be spring-powered airsoft replicas by Cybergun, as this is the only airsoft 4506 available.
EDIT I believe these are earlier generation 645.
G2000
A G2000 CO2 pistol is seen in the pistol case at the gun store.
A Colt 1908 Pocket is seen on a rack in the pawn shop, likely the infamous and cheap "Colt 25" airsoft gun.
Colt Double Eagle
A Colt Double Eagle is seen on a rack in the Hammerson factory. All three pistols on that row are airsoft replicas made by Tokyo Marui.
Ruger KP90
A Ruger KP90 is seen on a rack in the Hammerson factory, airsoft replica.
Smith's Hand
Smith (Clive Owen) takes bullets from a Para-Ordnance Nite-Tac and holds them between his broken fingers and then sticks his hand in the fireplace, shooting Hertz (Paul Giamatti) eight times. Since we only see four bullets in his hand, we realize they can't even get the capacity of the human hand correct! Some questions are raised: How does he eject the magazine when he can barely pull the trigger? How does he remove the bullets from the magazine with broken fingers? And does having all your fingers broken make your hand numb to all pain?
The general idea of the film, however, was to be as outrageous and unbelievable as possible, so none of the errors really matter.
Remington 700P
Mr. Hertz (Paul Giamatti) tries to kill the baby at the playground with a Remington 700 outfitted with a HS Precision stock and folding bipod, a heavy stainless barrel, as well as a stainless 3x9 stainless Buschnell scope. Based on the bolt length, it is assumed to be a .308 caliber.
Heckler & Koch G36C
Smith (Clive Owen) is seen loading up a Heckler & Koch G36C in the Hammerson factory, and another is seen resting on a rifle rack.
A SIG SG 552 rifle is seen on a rack in the Hammerson factory.
Heckler & Koch MP5A3
Smith uses several Heckler & Koch MP5A3s with tactical tri-rails as make-shift Rube Goldberg-esque traps to take out Hertz's men in the Hammerson factory.
An unknown CAR-15 variant is used by Smith (Clive Owen) as another Rube Goldberg-esque trap, in which he uses strings and trip wires to fire them.
IMI Micro Galil
Several of Hertz's men fire IMI Micro Galils at Smith (Clive Owen) during the car chase shootout. Smith knocks out one of these shooters with the door of his BMW and then takes his Micro Galil, using it to take out the pursuing vehicle.
Unknown AK variant
Multiple AK rifles are seen on a rack in the pawn shop but are too close and blurry to tell make and model. These are probably airsoft rifles, seeing as they aren't used in any of the shooting scenes and would probably just be expensive props if they were the real deal. They might also be Norinco copies, which don't cost very much either.
M16A2
Several M16A2 rifles are seen on a rack in the pawn shop.
Micro Uzi
A Micro Uzi is seen on a pistol rack in the pawn shop.
Mini Uzi
Several of Hertz's men are seen using Mini Uzis during the assault on Smith's house. Smith takes one of these and uses it to kill several thugs before switching to another gun.
Smith (Clive Owen) takes a Mossberg 590 "Compact Cruiser" off of one of Hertz's thugs and uses it to kill another, which unrealistically sends the thug flying in the typical action movie flair.
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Mossberg 500 "Cruiser"
Smith uses a Mossberg 500 "Cruiser" with an extended magazine tube as one of his traps, tossing a tutone Glock on a trip wire to kill a thug with it. When the thug is shot, the director imitates the camera blood splatter technique used in videogames.
Remington 870 Police Magnum
Smith uses several Remington 870 Police Magnums with synthetic stocks as trip wire traps in the Hammerson factory.