Nice job, Michael!
My understanding is that anything that retards or stops the gun from moving backwards will impede the inertia system so, if you put the butt against a concrete wall (I've never done this) and fire it, the action will not cycle. Conversely, if you hold it away from your body, (I have done this) it will cycle just fine. So, adding weight might impede the cycling but "limp shouldering" will not.
The M4 is gas-operated out of USMC's concerns that all their add-ons might prevent an inertia action from cycling.