I was just watching a video on youtube where a guy was modifying his overseer's guardian and he had stock and barrel options such as the machinegunners stock, and machine gun barrel. Where do I get these? Is it a mod or something else?

There is a mod that lets you apply any mod to any weapon, though as far as I'm aware, it still differentiates between melee and ranged weapons. Not sure how Unarmed weapons are affected.