Reportedly invented by American fortification superstar Joseph Totten (1788-1864) was a devilishly clever iron protective shutter system that saved American gunners from the potential embarassment of being shot while loading their weapon.

These were carefully-balanced shutters that remained closed until the cannon was fired: The gases emitted by the cannon shot before the projectile came jetting out of the gun's muzzle were supposed to SWING those shutters wide open, then the shutters would clang shut again once the projectile had been sent on its way.

Did this unlikely-sounding invention work? Probably, because lots of American forts had these things installed in the mid-19th century. What we're looking at here are the holes into which these shutters would have been locked.