The main protagonist of the film, FBI ASAC (Assistant Special Agent in Charge) Anthony 'Hub' Hubbard (Denzel Washington) and several of his fellow Agents carry SIG-Sauer P226 pistols as their standard sidearms.
Heckler & Koch MP5A3
During the bus incidents, N.Y.P.D. ESU officers can be seen carrying Heckler & Koch MP5A3 submachine guns fitted with Surefire forearms and flashlights, as per norm with SWAT-issue MP5s. FBI HRT members can also be seen carrying MP5A3s fitted with laser pointers and red dot sights during the raid on the terrorist safehouse.
Curiously, the FBI's MP5A3s have the "Navy"-style lower receivers and Surefire forearms when they are advancing up the stairs, but when they enter the terrorists' apartment and begin firing at them, the MP5s suddenly have the older-style SEF lower receivers and lack the Surefire forearms. It is likely that the MP5s used before entry are airsoft replicas.
Heckler & Koch HK94A3 (mocked-up as MP5A3)
During the raid on the terrorist safehouse, at least one of the MP5A3 submachine guns used by an FBI HRT member is actually a Heckler & Koch HK94A3 carbine mocked up as an MP5A3 (judging by the all-metal lower receiver).
Savage 10FP
During the incident with the second bus, N.Y.P.D. ESU marksmen can be seen getting into position with Savage 10FP sniper rifles fitted with Harris bipods.
Glock 19
The N.Y.P.D. officers throughout the movie carry Glock 19 pistols as their sidearms. Some of the FBI and CIA agents also use them.
IMI Uzi
During the incident at the schoolhouse, the terrorist holding hostages can be seen wielding an Uzi as he paces about. Despite having the responding police and FBI personnel out-gunned, he's still easily taken down when Agent Hubbard catches him off guard. One of the terrorists in the safehouse raid is also seen using an Uzi. This is a slight cultural irony, as the Uzi is primarily produced in Israel, and is being used by Arab terrorists.
AKM
When the FBI raids the terrorist safehouse, one of the terrorists engages the FBI in a shootout using an AKM assault rifle, though is inevitably taken down, even shooting one of his own comrades when he strays into his field of fire. As with most of the AKMs used in Hollywood, this is most likely to be a full-auto conversion of an Egyptian-made Maadi ARM.
Remington 870
Both the FBI agents and many N.Y.P.D. officers are seen carrying Remington 870 shotguns fitted with top-folding stocks. One of these shotguns is wielded by Agent Frank Haddad (Tony Shalhoub) during the hostage situation at the school.
Ithaca 37
Another weapon wielded by many of the FBI Agents is the Ithaca 37 riot model. Agent Frank Haddad (Tony Shalhoub) carries one of these shotguns during the raid on the CIA safehouse.
M16A2 Rifle
The M16A2 assault rifle features prominently in the movie; almost every soldier involved in the occupation of New York City is seen carrying one. A notable point in the film is that when Maj. General William Deveraux (Bruce Willis) is describing what the occupation will entail, he distinctly mentions the "M16A1 assault rifle", though the U.S. military had switched to the M16A2 years prior to the events in the film, and indeed the M16A2s are by far the most common rifles in the movie. It is also possible that many of the prop weapons used in the movie are semi-automatic AR-15 A2s, standing in for the M16A2 (since they're never fired).
MGC M16 Replica
It appears that some MGC M16 replica assault rifles are used by the U.S. Army soldiers in the movie. Some may be modified to look like M16A2s by adding A2-style handguards and A1-style birdcage flash hiders, but it's hard to tell.
XM177-type Carbine
Throughout the film, it is possible to see many soldiers carrying XM177 look-a-like carbines. They all have long, bulky flash hiders, which tends to characterize the civilian copies of the XM177, like the one seen below. So these are probably some semi-automatic copy standing in for the military-spec XM177E2.
M203 Grenade Launcher
Throughout the movie, it is possible to spot an XM177-type carbine fitted with an M203 grenade launcher in the hands of soldiers. It appears to be the exact same prop weapon being reused multiple times in different scenes.
M60 Machine Gun
In several scenes, Humvees and Cadillac-Gage Commando APCs can be seen with M60 machine guns mounted on them, most notably when General Deveraux is giving his press conference about the declaration of martial law and when the Army crashes the FBI raid on the Redhook chop-shop.
Browning M3 Heavy Machine Gun
In the scene where the Army opens fire on the Redhook chop-shop, one of the Commando APCs can be briefly seen with a Browning M3 heavy machine gun on the roof.
Beretta 92FS
Another weapon used by the Army is the Beretta 92FS pistol (standing in for the military M9). One soldier can be seen holding one on the FBI Agent that the soldiers mistake for Hubbard, and another is presumably the weapon General Deveraux uses to kill Tariq Husseini (Amro Salama). Also, it's difficult to tell, but it appears that the terrorist in the safehouse who receives the pizza also has a Beretta lying on the table.
Colt 9mm SMG (RO635)
During the opening scene in which a team of American black ops soldiers capture the Sheik, they use Colt 9mm submachine guns (RO635) which appear to be fitted with C-More sights. For some reason, the Colt SMGs sound suppressed when they are fired, even though they clearly do not have sound suppressors attached.
M26 Hand Grenade
During the raid on the Redhook chop-shop, one of the terrorists inside uses the distraction created by the arrival of the Army to pull an M26 hand grenade, the detonation of which provokes the soldiers outside to open up on the shop with everything they have, Agent Hubbard and Tariq Husseini barely surviving the onslaught.