I'm about to guest as a battlesmith in a campaign and decided to use the Arcane Propulsion arm (because cool robot arm) but I can't seem to figure out how to use it as a weapon? The item itself is a wonderous item, would an Arcane Arm need to be homebrewed?

Thanks for the help. Sorry but I'm looking for the Arm, not the armour (annoyingly similar things). I've got it equipped and attuned, but it doesn't show as an attack.

You'd need to create a Custom Action for it, or else homebrew a weapon.