U-691 is topheavy with weaponry, armed to the teeth because of the aircraft threat. Bischoffs crew opens fire on the destroyers with all of the small and medium-sized stuff, to give the deck gun crew a chance to line up its shot. At this range, the danger is that the shell will pass all the way through the destroyer’s hull, and out the other side, without detonating. You have to be patient, take your time, aim for the engines. Bischoff’s crew knows this.