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 (I known 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. Bill Hertz (Paul Giamatti) uses a brushed chrome Magnum Research Desert Eagle Mark XIX chambered in .44 Magnum 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 interigates 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 interupts 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 infact the .44 Magnum model holds eight shots plus one in the chamber. For that matter, none of the Desert Eagle variants hold six shots, including the largest .50 AE model, which holds seven rounds plus one.
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
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.
Special
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.
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 also prominently seen armed with two of these tutone USPs on the film's poster.