Heavily modified Beretta 92FS pistols are used by John Preston (Christian Bale), Andrew Brandt (Taye Diggs), DuPont (Angus Macfadyen), Errol Partridge (Sean Bean), and the rest of the Grammaton Clerics throughout the film. The versions used in the movie were converted to automatic with a drop-in auto sear (DIAS) and given longer barrels and modified slides. The non-firing versions also had a selector switch on the side (which does not appear on the firing or "practical" weapons).
The weapons are definitely the 92FS and NOT, contrary to popular belief, the Beretta M93R. The M93R is based on the older model 92-series pistols and still has the frame-mounted safety (whereas the weapons in this movie clearly have the slide-mounted safety) and a folding grip attached to the trigger guard (missing from the weapons in the movie). Although the 92FS is not normally a fully-automatic firearm, conversions are possible with DIAS units and have been done for many Hollywood movies (including Judge Dredd and Underworld).
The Heckler & Koch G36 is used by many Sweepers throughout the movie, and by Brandt (Taye Diggs) during one of the raids. Preston (Christian Bale) also briefly uses a G36 to kill several guards at the end.
The Walther WA 2000 is used by a heavily armored Sweeper during one of the raids. Although the weapon is a semi-automatic sniper rifle, it is wielded as though it were a shotgun; the scope and bipod have been removed, and the actor playing the Sweeper pretends to rake a non-existent pump action under the barrel (accompanied by the sound of a shotgun being cocked). The WA 2000 in this movie is also a replica of some sort. It doesn't carry any trademarks at all.
A Remington 870 with no stock and an Aimpoint is used by the resistance leader Seamus (Dominic Purcell). Some of the Police are also using Remington 870s fitted with flashlights and forward grips. During an altercation with several Sweepers, Preston (Christian Bale) gets a hold of two 870s and uses them to kill their owners.
An M16A2 is briefly seen in the hands of a Sweeper whom Preston (Christian Bale) attacks when his wife is arrested which he drops immediately before snatching an AR-70 from another sweeper, commonly miss attributed as a continuity error though it is simply seen as one since the manuever is so fast. When Preston is in the Warehouse raid whipping a group of Sweepers - they use AR-70s, but in some shots, they all clearly have M16A2s instead.
FN P90s are used by the Sweepers in white suits; these particular weapons are clearly rubber props, as evidenced by the fact that even the magazines (which are transparent on a real P90) are solid black. Kurt Wimmer mentions on the DVD commentary that originally, all of the Sweepers were supposed to use the P90, but the Belgian government did not supply the real live-fire P90s it had promised to the production, only rubber training versions. Most of the weapons used by the Sweepers are, therefore, guns that were loaned by the German government (including the H&K G36s, H&K MP5s, and Walther MPKs).
Most Armored Sweepers and police in the movie use Glock 17 pistols as their sidearms. Mary O'Brien (Emily Watson) manages to grab one of these pistols from a Sweeper and tries to kill Preston with it before the Sweeper retains control of his weapon.
The Uzi submachinegun can be seen being wielded by a resistance member in the ending when the resistance rise up and overrun the Librian Government buildings.
The Sten can be seen at the end of the film along side the Beretta M12 storming the Government, this can be an understandable firearm for the resistance to have as it is relatively simple and was made by hand by resistance groups in WW2.
On the DVD front cover, an edited image of Christian Bale and Taye Diggs shows them both holding duel Glock pistols that have extended frames and slides. They are likely digital fabrications.
