The Auto 9 is the name of Robocop's sidearm in the Robocop movies, the one that he keeps inside his leg. Although this name was never mentioned in the film, it is mentioned in the script as well as video games and toys based on the film.
The gun was fabricated from a Beretta 93R, which can be seen unmodified in the film Broken Arrow, being used by John Travolta during the train sequence. It looks like a regular Beretta automatic except there is a small fold down grip on the trigger guard for the shooter's other hand. This is because the gun fires a three round burst and packs a wallop in recoil. The auto-9 verion of the gun loses this grip, because Robocop is such a badass that he can fire it with one hand. Viewers who compare internet photos of the auto-9 compared to the 93R notice the biggest change is the giant twelve inch barrel on the auto 9 with heat sink grooves along the side and recoil porting at the end so that flames shoot out the sides in a very cinematic way.