Hydra Shotgun
To get the Hydra Shotgun you need to beat the game on normal or higher difficulty.

Infinite Grenade Launcher
To get the infinite grenade launcher you need to beat the game in Hell mode.

PC356 Handgun
Once you kill 150 enemies you'll unlock the PC356 handgun.

PSG1 Rifle
Once you scan 15 hidden prints you'll get the PSG1 rifle.