I'm leaving out the obvious examples like the MP-5 TX, Ingram Smartgun, etc.
The original Ares Predator is modelled after the gun Robocop uses, which in turn is a modified Beretta 93R.
The Assault Cannon class is inspired by the Cobra Assault Cannon, also from Robocop, a modified M82 Barrett.
The Vanquisher and Vindicator cannons are inspired by the GAU-8 Avenger of the A-10 Thunderbolt II.
The Walther Palm Pistol looks like the "Bladerunner Blaster".
The TMP Submachine Gun looks like a Tec-9.
The LAV-98 "Devil Rat" APC is a M113.
The LAV-103 "Striker" Light Tank is a M2/M3 Bradley IFV/CVF.