Rihanna -- Shooting Up Sin City

Rihanna did her best impression of Rambo Thursday night -- when the singer let loose with some FULLY AUTOMATIC MACHINE GUNS ... and TMZ has the crazy footage.

RiRi rolled up to the Guns and Ammo Garage in Las Vegas for a midnight session at the gun range -- where we're told she shot a variety of fully automatic weapons, including the SAW machine gun and the M4 and AR-15 rifles.

You gotta check out the video ... because who doesn't love watching a hot chick shoot a gun???