Yes, apologies, my reference to a door-gun was a 'mis-speak', to coin Hilary Clinton. The issue of the rotors being a problem is covered in the story, as is the possibility of utilising C130-style pallet loads.
Given the V-22's tiltrotor configuration and nearly 12 m diameter
rotorblades, forward-firing munitions could not be carried on underwing
hardpoints. Instead, the aircraft would either have to employ ramp-mounted or cargo bay-stored canister munitions, similar to those carried by the USMC's KC-130J Harvest HAWK Hercules gunship, or sponson-mounted stub-wings.
A side-firing cannon/machine gun could be fitted, but this would involve some structural re-modelling as the V-22 does not have a paratrooper door on the left-side aircraft fuselage, and the door on the right side is located forward of the rotors, which would present safety issues for the aircraft.