The Autocannon is a heavy weapon firing explosive large-caliber rounds that penetrate light vehicle armour and shred infantry with ease. The weapon is generally restricted to vehicle mounting or dedicated heavy weapons teams, although a Space Marine would be capable of using one like a light machine gun.
In Chapter 16, Captain Titus can use (and pick up) a mounted Autocannon. The weapon has unlimited ammunition as long as it is mounted, switching to limited ammo if it is torn off the mount.
In addition to decent accuracy over long range, the Autocannon is an explosive weapon with a suprisingly high rate of fire. It will overheat if used continuously, in the same manner as the Heavy Bolter.