The most common sidearm seen in used by the SG teams (although in real life, the M9 is the military designation for the Beretta, the prop weapons used in the show are most likely the civilian Beretta 92FS).
Luger P08
A Luger P08 is used by a member of the Lucian Alliance and later Teal'c in "Camelot" (Season 9, Episode 20).
Taurus Model 85
Vala Mal Doran (Claudia Black) can be seen pulling the dressed up Taurus Model 85 from I, Robot on a Jaffa trader in "The Ties That Bind" (Season 9, Episode 4). This marks one of two appearances, the second one being in "Dominion" (Season 10, Episode 19).
Submachine Guns
Calico M955A
A Calico M955A with 100 round helical magazine appears in the hands of a Lucian Alliance guard in "Off the Grid" (Season 9, Episode 16), equipped with what seems to be a laser aiming module.
SG-1 were first seen using MP7s when impersonating non-Earth humans, rather than their more distinctive FN P90s. Since then, the MP7 has not been seen in used by the SG teams and only a handful of extras are seen holding them.
MAC-10
In the episode "Off the Grid" (Season 9, Episode 16), Worrel (Vince Corazza) uses a MAC-10 for a couple of scenes before taking one of SG-1's MP7s. The barrel attachment is of a similar build to the one used on Blade and Blade II.
Steyr SPP
A thug in the episode "Off the Grid" (Season 9, Episode 16) holds a modified Steyr SPP pistol with factory rails, Aimpoint sight and custom flash hider.
Shpagin PPSh-41
Seen in "Ethon" (Season 9, Episode 15), in the hands of Caledonian soldiers. They apparently favor the curved box magazine rather than the familiar drum magazine.
Shotguns
Cobray Street Sweeper
Seen in the hands of Cameron Mitchell (Ben Browder) in "Stronghold" (Season 9, Episode 14) during the attack on Ba´al´s Ha´tak.
Rifles
Diemaco C7/Olympic K4B
Seen used by Marines, and the security of Stargate Command. These are all Olympic Arms K4B rifles (same receiver style as the Colt Model 715/Diemaco C7), standing in for the M16A2.
GAU-5/A and GAU-5A/A
The GAU-5/A and GAU-5A/A were the USAF contract versions of the XM177 and XM177E2 "CAR-15" short assault rifle. The USAF Security forces used them until recently, however, most of the non firing GAU-5A/As seen in the show were simply XM177E2 "Airsoft guns". The actual GAU-5A/A carbines look like the XM177E2 but don't have forward Assist buttons.
The GAU-5 variants that appeared on the show seem to be a mix of full-auto converted Colt AR-15 SP1s and Olympic Arms K3Bs, all with chopped barrels and fake GAU-5 flash hiders installed. The converted AR-15 SP1s have the "slab-side" receiver, while the K3Bs are recognizable by their 715-style receiver (forward assist, brass deflector, A1-style rear sight) and "Stowaway" pistol grips.
Heckler & Koch G36K
Two versions of this gun were seen throughout the running of SG1. The true "K" version with the scope and the "KE" version lacking the scope.
M16A1
Usually seen carried by the Cheyenne Mountain guards in stock footage.
M4A1
One of the primary weapons utilized by the Stargate teams. During later seasons they are rarely seen without any attachments, mostly being equipped with flashlights, Cobray flare launchers and laser aiming modules. A small side note for the interested reader: The lower handguard on the M4 has to be removed in order to accommodate underbarrel grenade launchers like the M203 or, in this case, the Cobray 37mm Launcher.
Machine Guns
Browning M2
These two guns were set up in the gate room but were never used in the 10 years the show ran for. The M2 was also used at the Alpha site in "Allegiance" (Season 6, Episode 9), but was - once again - not fired. The weapon was then seen in "Bad Guys" (Season 10, Episode 16), mounted on an APC used by the alien security forces. The only instance were the weapon saw actual combat was in "Forever in a Day" (Season 3, Episode 10), were it was mounted on a F.R.E.D. during the onslaught of Heru'ur´s Jaffa.
Daewoo K3 (mocked up as M249)
Contrary to popular belief, there are no real FN Minimi or M249 light machine guns used in Stargate: SG1 or its sister show, Stargate: Atlantis. The light machine guns used by the SG teams are actually Korean-made Daewoo K3s that have been fitted with M249 Paratrooper-style stocks and shortened barrels. Some of the K3s seen on the show also have RIS rails and vertical grips similar to those seen on the Mk 46 Mod 0.
M60 Series GPMG
Seen used by Marines in a support role in several episodes. Some marines are carrying the M60E3 version.
Heavy Weapons
M18A1 Claymore
Frequently used by the SG teams in order to lay traps. Usually used in conjunction with a remote detonator, much like the C4.
M61 Vulcan
In "Collateral Damage" (Season 9, Episode 12) Mitchell (Ben Browder) is briefly relieving a memory of his earlier Air Force service. During the sequence he flies a F-16 during a bombing run in an unspecified conflict, which is armed with a M61 Vulcan.
Cobray CM203 Flare Launcher
Seen attached to M4s and M16s. Prop wise, these were all Cobray 37mm flare launchers meant to mimic the appearance of an M203. What tips them off is the absence of a second trigger, the shape of the trigger guard, the lack of a step up in the receiver, and the cocking lever on the SIDE of the receiver.