The very first handgun you collect in the game, the "TechLite Arms H662 9mm Pistol" seems to be based somewhat on a Walther P99, with a slide-mounted safety. The reloading animation shows Decker dropping the magazine, inserting a new one, and pulling the slide, regardless of you whether or not the weapon was actually empty when the weapon was reloaded.
Desert Eagle .50AE
One of the only guns in-game not to have some sort of made-up name, the Desert Eagle is the second most powerful weapon in the game, able to take out most enemies in two shots.
Sub-Machine Guns
"SMP-9 Submachine Gun"
The only SMG in the game, this frankengun seems to be an amalgamation of a SG 552 with the magazine of a UMP-45. While it's not the strongest gun in the game, it is prevalent throughout the entire campaign due to every soldier using one.
Assault Rifle
"XT9-MU Assault Rifle"
Another frankengun, the "XT9-MU" appears to be built around a Heckler & Koch G36 with what appears to be the front end of a SPAS-12. The gun also doubles as a Sniper Rifle due to being the only gun in the game to have a scope on it. Unlike the other guns, the reload animation is very simple: Decker drops the empty magazine and inserts a new one, but doesn't bother to chamber the gun.
Shotguns
"Marauder Model 1010 Shotgun"
The only shotgun in the game, the "Marauder Model 1010 Shotgun" is the third most powerful weapon in the game, able to decapitate werewolves at close range in one shot. This weapon is also seen in Decker hands during most of the later cutscenes.
Heavy Weapons
M249 Squad Automatic Weapon (SAW)
The second gun to not have a name change, the M249 the only LMG that one can acquire in the game.
"Anti-Air Missile Mark 7"
The "Billburg-Osterman Tech Shoulder Launched Anti-Air Missile Mark 7", aside from having the longest name, seems to be somewhat based on the SMAW and is the most powerful weapon in the game, able to destroy all enemies in one hit, including the Gryphons. It is able to be free-fired or fired via a lock-on signal.