The All Terrain Armored Cannon, or AT-AC, contained one chin-mounted cannon, a large cabin surrounded by armor plating, and a forward view slit along with two large windows equipped with dual-barreled blasters.
Around 18 BBY, at least two AT-ACs were deployed by the Imperial Advanced Science Division on Ord Mantell during a raid to capture the rogue clone Omega of Clone Force 99. As the AT-ACs patrolled the streets near Cid's Parlor, where Imperial clone commandos were conducting the raid, one of the walkers was hijacked by Echo and AZI-3. Echo used the walker to ambush a Low Altitude Assault Transport that was being prepped to ship his captured squad members off-world. In response, a second AT-AC destroyed the hijacked walker by shooting at its reversed knee brace, causing it to topple over. The AT-AC continued to fire at the Bad Batch as the rogue clones fled the scene.
The AT-AC walker is described as the "next evolution" towards the AT-ST.