Typically, RoboCop fires this weapon in 3-round burst mode, but the gun also had a seven round burst and a fully auto version made. The fictional stats of the weapon claim it has an implausibly huge 50-round magazine while the real life version 93R and the RoboCop pistol only used 20 round magazines. Weapons master Randy Moore also made new ratchets for the 93R which allowed them to shoot 3 shot bursts, 7 shot bursts and full auto. With the 7 round burst you could get 3 trigger pulls before reloading. The slide lock was disabled so the pistol wouldn't stay open after it was empty.
Although it is a common movie flaw that action heroes never reload their guns, RoboCop can actually be seen reloading two times on screen. Once in the first movie towards the end and another time in the second movie at the beginning.However both Paul Verhoeven and Irvin Kershner hated the idea of a RoboCop reloading his gun actually being shown. Paul felt it was a good action point. Once. Kershner absolutely did not want a reload and the only way weapons master Randy Moore managed to get it in was having Robo drop the empty magazine and move across the room and reload. So to cut the magazine change would have caused a movement jump cut and Kershner had to leave it in on the final edit.
According to the RoboCop cast & crew the production team had to fill out extra paperwork to even allow the gun into the United States, because while technically a "pistol", for all intents and purposes it is a high-caliber near-automatic weapon, which comes with extra legal restrictions. In addition to the firing guns a variety of stunt "dummy" ones were used for non-firing shots (for safety reasons). These "dummys" were made in rubber.
Of course the gun in reality, when fired, sounds nothing like the sound which echoes through the movie. Such is the magic of post production sound editing. The finished gun sound was made up of a variety of real noises.
The reason RoboCops gun is called Auto-9 is not certain, it could be because it's an automatic pistol & the bullets used are of 9mm in diameter.It could also just as well be the 9th automatic pistol OCP has patented (similar to Glock's naming system of 17, 18, 19, 20, etc.). It is also common practice when a company is developing a product to have an earlier seperate prototype name before marketing can find a suitable final name, this would explain why such a interesting distinctive pistol would have such a bland name. 
For RoboCop: The Series and RoboCop: Prime Directives, the Auto 9 was shown as being able to carry special ammunition (mainly armor piercing in Prime Directives and Non-Lethal in The Series). The series also introduced that the auto-9 is encoded and can only be fired by Robo himself.The gun on the RoboCop: the series is a lighter version of the one in the first movie. They created a lighter gun when importing the real gun into Canada proved difficult. It was also easier to handle by the actor in the Robo-suit. The Auto-9 name is never mentioned in any of the movies but it was actually referred to as such in the original script. The name then made its way from there into promotional materials, games, comics etc. While filming the movies it was only ever called the Robo pistol by cast and crew. In addition to its appearances in the RoboCop films and TV shows, the Auto-9 has also had cameo appearences in several other movies, including Sin City(2005), City Hunter(1993) and Dracula 3000(2004). In the promotional photos for the 2014 RoboCop reboot the Auto-9 is heavily featured but in the actual movie the gun never actually show up.
