The Ironhelm Thunderhammer has the ability to cleave through units with the charged special. The expected intention is to swing through crowds to slam an elite, a problem arises from the heavies having way too much mass limit to land on specials and elites, and no way to go straight into the only low mass overhead the weapon has.
Luckily there is already a mechanic to fix this, behold
Weapons can have modifiers applied to specific attacks that treat armour types as having more or less mass; If the heavy swings are given increased mass mod to flak or maniac it will ensure that the discharge can always land on the important specials.