Iron spikes were a part of a battery's equipment. They were
long enough to reach through the thickness of the barrel and extend into the bore. Made of hardend wrought iron with a soft tip.These spikes were headless, so a claw bar could not be used to try and remove the spike.
When it became apparent that a gun was going to be lost to the enemy, with no immediate hope of relief from the infantry support,the gunner or whomever was in command of the gun would attempt to render the gun inoperatable.
One method of accomplishing this was to take one of the spikes and hammer it down the vent hole (vents were made of copper) and then take a cannon ball or shot and ram that down the bore (with no powder charge) until it wedged into place by bending the soft tip of the spike.
Once spiked, the entire vent had to be removed and replaced or a new vent hole drilled to make the gun servicable again.
(after the wedged shot was removed too).