From Mud and Blood official Wiki
|Effect||Allows a unit to pierce the armor of certain vehicles, like tanks.|
|Example(s)||M3 AT Gun, PAK-38, Bazooka, Tank, 75mm AT Gun (Halftrack Upgrade)|
|Description||Also called Anti Tank (AT). Units with this specialty are specifically designed to take out armored target such as tanks or any vehicles where the crew is entirely shielded from small arm fire. A unit that is AT is likely to be slow firing and crewed. An AT weapon is, by nature of its ordinance, usually less capable versus infantry than a weapon specifically designed for targeting dismounted troops. Examples include the M3 AT Gun and the Bazooka.
The main difference is the concentration of power. If you compared, say, an arty round to the round of an M3 AT Gun or to that of a PAK-38, the round fired by the Anti-Armor vehicles would be smaller and have less fragmentation than the arty round, but they would have the same damage, as an AT gun can knock out even a Tiger Tank in only one hit.