What has been driven in can be pulled (or drilled) out ...
Back in the 1990s, Guns & Ammo (I think) ran an article about a fellow who had purchased a “demilitarized” French 75mm field gun. They had driven a mild steel ring into the chamber, preventing a round from fully seating and the breech block closing on it. He was able to reach in there with a hacksaw blade and cut the ring out, return the gun to firing capability, and reload cartridges for it. Reloading press was a modified log-splitter.
It depends how well it was done. Given the situation they must have been in a hurry.