The Mark 42 made the five-inch gun a practical backup

The Mark 42 five-inch/54 gave the Belknaps a surface and anti-aircraft weapon when a missile was unsuitable or unavailable. Its manual anti-aircraft position became impractical as attacking aircraft grew faster, but the mount remained useful as a conventional backup. At about 61.4 tons, it was much heavier than the roughly 22.5-ton Mark 45 that replaced it. The Mark 42 served aboard numerous US destroyer and cruiser classes, allied warships, and Japanese Maritime Self-Defense Force ships, while a gun from HMAS Brisbane was preserved ashore.