What is a breeching gun?

Answer 1

Term is breaching- meaning to open. Shotguns used for the purpose of forcibly opening a locked, barricaded door by blowing off the hinges or lock.

Answer 2 Alternately, during the middle ages, when cannons were first being used during seige warfare, some armies were equipped with guns of very large caliber (some up to 20" or more) that fired very heavy stone or iron projectiles. These huge guns were intended to batter down the stone walls of the fortresses of those days. I have heard these guns referred to as "breeching guns", also.