The lead robber during the beginning of the film, holds a Italian made Pardini PC9 pistol. It is a large match grade target pistol, used in European IPSC style competitions and somewhat unlikely weapon to be carried by a bank robber.
A Beretta Cougar Inox with wood grips is used by one of Kwai's bodyguards. A thug earlier at the house also appears to be using the same Beretta Cougar Inox.
Glock 17
A thug at the house uses a Glock 17. One of Kwai's bodyguards also uses one.
Walther P99
Seen inside Frank's fishing tub, also where he keeps several grenades and his SIG SG 552 and spare magazines.
Smith and Wesson 5906
On the film's cover, Frank has what looks like a Early Model Smith and Wesson 5906 in his left hand and the Beretta 92FS Inox in his right.
Smith & Wesson Model 29
Two of the bank robbers in the beginning of the film used Smith & Wesson Model 29 revolvers. One is blued and the other is bright nickel plated.
An Smith & Wesson Model 60 Snub nosed revolver (with black plastic grips) is carried by Tarconi (François Berléand) who offers it to Frank who uses it in his escape. Tarconi later offers the same revolver to Frank for his assault on the shipyard, which Frank declines, instead showing that he has bigger firepower for the mission.
Ruger SP101
The Ruger SP101 revolver is carried by the French National Police. Lai (Qi Shu) later uses one aswell.
Manurhin MR-73
At least one of the French police has a Manurhin MR-73 Gendarmie Revolver.
If you look closely, you can see the hammer and the cylinder lock look different, because the models used in the movie are Manurhin F1, the F1 has a Ruger frame and mechanism, Manurhin produces the barrel and the cylinder.
One of Wall Street's thugs at the house uses an MP5K to try to kill Frank.
Mini Uzi
Kwai (Ric Young) uses a silver (not nickel-plated) Mini Uzi during the highway chase.
Beretta M12
Some of the French National Police who open the truck's container have Beretta M12 Submachine guns slung.
Assault Rifles
SIG SG 552
A SIG SG 552 equipped with a weaver made late 1960's "single point" occluded-eye sight is used by Frank when he is checking shipping containers for smuggled Chinese people.
Steyr AUG HBAR
A Steyr AUG HBAR is used by one of Wall Streets' assassins to shoot up Frank Martin's house.
Customized M16A1
The third assassin who shoots up Frank Martin's house uses a M16A1. It appears to be a standard M16A1 with an unusual A2-ish forearm and a strange upper section that looks like it's trying to copy a Beretta AR-70/90 in general shape and style (especially the forward slant of the carry handle), it's possible this might be a very old school laser aiming device.
SIG SG 543
A SIG SG 543 is used by one of Wall Streets' assassins to shoot up Frank Martin's house. His SIG SG 543 has a 40mm launcher attached. The launcher for the SIG SG 550 series is the SIG GL 5140 grenade launcher however this is not one of those launchers. Maybe an older launcher meant for the SG 540?
Ruger AC556
During the battle inside the Bus depot, one of Wall Street's thugs fires a customized Ruger AC556 (the full auto version of the Ruger Mini 14) at Frank Martin. The rifle has a Choate™ aftermarket flash hider extension as well as a flashlight mounted to the barrel.
Steyr AUG
During the battle inside the Bus depot, one of Wall Street's thugs uses a standard Steyr AUG at Frank Martin.
M16A2
During the battle inside the Bus Depot, one of the assassins fires a full auto M16A2. (The real M16A2 is a three shot burst weapon, so this is a 'fake' M16A2, or an M16A3 (which was the A2 with the A1 fire control group that was issued in small numbers to Naval personnel). In some shots there are marks on the barrel that resemble the cutouts of an M4, however upon closer inspection, they are wear marks from either a non-standard laser or flashlight mount.
Shotguns
Remington 870
Wall Street during the highway chase uses a Remington 870 shotgun with black synthetic furniture, a pistol grip, and a flashlight attached under the barrel.
Mossberg 500
The luckless robber at the beginning of the film has a Mossberg 500 shotgun that has been tapped for scope mounts.
Other
Fake MATADOR Rocket Launcher
One of Wall Streets' assassins uses a mocked up launcher that somewhat resembles the MATADOR Missile Launcher, however, the method of launching is completely different. It loads (and reloads) in the rear like an M1A1 Bazooka and fires 'self guiding' rockets that appear to be able to make 90 degree turns in flight.
Later the man firing the launcher pulls out the 'Special' munitions - a standard M20B style rocket with a cartoon shark face painted on to indicate it's 'special status'.
Notes
During the attack on Frank's house we can see a lot of 'spent' rounds falling to the ground. The extended tip and star crimped ends indicate that they are 5.56mm blank ammo.