deculassement

from The Century Dictionary.

• noun In gunnery, the unbreeching of a cannon; any serious damage to one of the essential parts of the fermeture or breech-closing mechanism of a breech-loading gun.

from the GNU version of the Collaborative International Dictionary of English.

• (Ordnance) An accidental blowing off of, or other serious damage to, the breechblock of a gun; also, a removal of the breechblock for the purpose of disabling the gun.