The auto-cannon is an autonomous defense turret equipped with twin minigun cannons and relatively heavily armored. They are usually part of an automated defense system. Despite their fearsome araments, someone loaded up on Psycho can survive longer ranged attacks.
Auto-cannons are indeed fearsome adversaries, and are ever-watchful of their domain. Their dual miniguns spell doom for unwary player characters - especially those with low levels DT. There is hope, however, as auto-cannons are vulnerable - just like most other robots - to electro-magnetic pulses.
They can be found at military installations like Sierra Army Depot, armed with heavy dual miniguns loaded with 800 5mm jacketed hollow point rounds and the Enclave Oil Rig, outfitted with heavy dual miniguns loaded with 800 .223 FMJ rounds.