Just as the Mark 48 Heavyweight Torpedo is abbreviated to MK48, the Mark 45 Gun is abbreviated to MK45. It's an automatic 62 caliber gun which you'll find fitted to a number of different cruisers and destroyers, and can be turned on any target regardless of whether they're in the air, on land, or out at sea.
As many as 20 rounds per minute can be fired at targets almost 15 nautical miles away. It's apparently even more accurate than a guided missile.