From Lexicanum

An Assault Drill is a weapon used on Ironclad Siege Dreadnoughts. It drills through rock as easily as reinforced ferrocrete, which is the substance usually used when building bunkers. Once the drill has penetrated the wall, a built in Heavy Flamer unleashes an inferno inside the bunker, killing everything within. The drill consists of three ball-shaped grinders which rotate to produce a drill effect.^1-p29

Sources

• ¹:Imperial Armour - Apocalypse