As I understand the Inertia system, upon firing, the gun moves back along with the locked bolt leaving the bolt carrier to remain stationary until such time the bolt compresses the inertia spring causing the bolt carrier to direct stored energy rearward to cycle the action. Thus shouldered or unshouldered, the Inertia system will cycle.
That said, I believe if you can lock the gun from moving rearward upon firing, the Inertia system will fail to cycle. Now that said, I can't think of a realistic way of doing this aside from putting an M2 in a vice around the receiver and stopping all rearward motion with a very large mass under the vice. Very unlikely in a real world shooting scenario. This same theory does align well with the idea that mounting a lot of extra weight to the gun can reduce cycle reliability. Add enough weight and you essentially mimick my vice grips example.
There is an excellent writeup on the Inertia system on Wikipedia here.