A Beretta 92 (distinguished by its rounded trigger guard and butt-mounted magazine release) is used by Sergeant First Class William James (Jeremy Renner), most prominently when James stops an Iraqi taxi. Mark Boal, the film's writer, notes on the DVD commentary the gun (the 92FS) that was supposed to be used in the scene got stuck in customs, which explains why it changes to the correct Beretta 92FS later in the film. Boal goes on to say that acquiring the 92 seen here was a difficult task; he scoured local bars, trying to find a suitable pistol, with no success. Finally, he found a Jordanian General who gave the producers the used 92.
Beretta 92FS
For the rest of the film, James and Sergeant J.T Sanborn (Anthony Mackie) both use the Beretta 92FS pistol, as noted by the hocked trigger guard and slide-mounted safety.
Glock 19
What appears to be a 2nd Generation Glock 19 pistol is carried by the leader of the British private military contractors (Ralph Fiennes). Sergeant J.T Sanborn (Anthony Mackie) takes the contractor's Glock and holds it on his comrades while telling them to put down their weapons. It is possible that the pistol may be a Glock 23, but this is unlikely because Hollywood's armorers tend to use mostly 9mm pistols, since until recently 9mms were the easiest caliber to convert to blank-fire.
Rifles
M4A1 Carbine
M4A1 carbines are used by Sergeant First Class William James (Jeremy Renner), Sergeant J.T Sanborn (Anthony Mackie) and Specialist Owen Eldridge (Brian Geraghty), as well as by other American soldiers. The carbines are equipped with a variety of sights and other attachments. Note that throughout the film, their buttstocks interchange between the 3rd and 4th generation stocks (even in the same scenes). The sights and attachments often change too. In particular, for most of the movie, James doesn't have a light on his M4, but one appears on it during the rescue of Eldridge. Also, in one scene, Sanborn's ACOG scope is briefly replaced by a red dot sight, before switching back. A M4A1 is also used by one of the British contractors, although it is never shown very clearly.
Airsoft M4A1 carbines
M16A4
A few American soldiers carry M16A4 rifles (possibly airsoft replicas, by the unnecessary screws in the A2 stocks), fitted with reflex sights and other attachments.
Kalashnikov Variants
AKM
AKM rifles are carried by Iraqi National Guard soldiers and insurgents.
MPiKMS 72
A MPiKMS 72, an East German version of the AKMS with plastic handguards and a side-folding stock, is seen slung over the shoulder of the Iraqi police officer that talks to SFC James about the car bomb at the UN compound.
Customized AKMS
Customized AKMS rifles with RIS foregrips and reflex sights are carried by most of the British private military contractors.
Barrett M107
A Barrett M107 (not a M82 as noted by the full-length accessory rail) is used by the private contractors to respond to an insurgent ambush, and is later used by Sergeants James and Sanborn to shoot several of the attackers. It should be noted that Sanborn and the other EOD members could conceivably know how to operate a Barrett since it can be used to detonate IEDs as an anti-materiel rifle.
FPK/PSL Sniper rifle
A FPK / PSL Sniper Rifle is used by an insurgent sniper who ambushes the EOD team and the private contractors in the desert.
Machine Guns
Browning M2HB
Browning M2HB machine guns are mounted on top of U.S Army Humvees, including that of the EOD team. It is also used by a British contractor during the ambush.