The Mk45 5”/54 gun was, until recently, the standard in production medium caliber gun for US Navy warships, being replaced in production (but not in existing fleet units) by the Mk45Mod4 5”/62 gun, which is mechanically almost identical, but has a longer barrel for greater range. It fires the same ammunition as the earlier 5”/54.
Here’s an older video, dating from the early 1990s showing a demonstration of the lethality of proximity fused rounds against simulated truck type targets.
The “HECVT” projectile stands for High Explosive Common Variable Time. That is, it’s a high explosive shell. Common means it is a general purpose round, as opposed to having an armor piercing body, or prefragmented anti-aircraft body. It is, by far, the most common round- hence, Common. Variable Time oddly doesn’t stand for Variable Time, but instead for Proximity fused. That is, a small radio transmitter in the nose of the fuse senses when the round is within a predetermined distance from an object, and then initiates the bursting of the charge. Variable Time was a cover story from when VT was invented in World War II to keep the Germans or Japanese from discovering how VT worked, and either developing their own, or countermeasures to defeat it.