The AR-18 is a gas operated, selective fire assault rifle chambered for 5.56x45mm ammunition. he gun is used with the stock removed (like the SPAS-12) and with two magazines taped together in a 'jungle style' configuration. He uses it primarily during the assault on the police station and during the car chase following the motel scene. The script refers to the rifle as an AR-18 and it is shown as such in the movie, though rather obviously a California gun store in 1984 would never sell a fully automatic rifle over the counter. The Terminator novel excuses this by stating the T-800 instead took a civilian AR-180 semi-automatic rifle and converted it to full-auto using tools stolen from a hardware store earlier in the day.