A hull that sticks in the chamber of a pump or semiauto locks the gun tight. It's a depressingly frequent occurrence with today's budget guns, which may have poorly finished chambers and may not be built to exacting tolerances; and modern ammo with steel bases that stick more frequently than brass. If you're shooting a pump and your life or next meal depends on a follow-up shot, perform the mortar drill. Hold the gun by the forearm, muzzle up, and rap the butt on the ground. That should dislodge the hull. However, this can potentially bend the gun's action bars, so if it's not an emergency situation, unscrew the magazine cap, hold the gun by the grip, and pull the barrel off to dislodge the shell that remains behind in the grip of the extractor. I've had to do this to others' guns many times at high school trap practice and in a duck blind.