You won't mess up anything permanantly, but you will fill the gas tube and action with bullet lube and lead. You're not likely to get a load to cycle the action and have any accuracy either. I shoot cast exclusively in my AR15, but it has the gas port plugged, upper is slotted for a bolt handle, bolt carrier drilled and tapped, and has a handle on the left side. No recoil spring or buffer, it's strictly manually operated and shoots cast great! But I wouldn't mess with cast in an auto.
