The U.S. Navy is buying aircraft type rotary machine-guns to use in defending their warships against attacks by multiple small boat attacks (especially small boats full of explosives.) The guns in question are lightweight Gatling gun (multiple rotating barrels) type weapons introduced by the U.S. Air Force in the late 1950s. The U.S. Army developed, and widely used, a 7.62mm version on helicopters during the Vietnam war. This weapon weighed only 35 pounds, carried 2,000 rounds of ammo and had two rates of fire (33 or 66 rounds per second.) Because of the high rate of fire, these weapons had an effective range of 1,500 meters. The navy version, the GAU-17, has 4,000 rounds of ammo. The weapon can actually fire up to a hundred rounds per second. But lots of 7.62mm rounds are not going to provide sufficient range, or hitting power, to stop the larger speed boats that might be used. Moreover, the light weight of the GAU-17 is not a critical factor for ships. So the navy is also mounting more .50 caliber (12.7mm) machine-guns. These range from dual gun mounts (weighing in at about 250 pounds) to the three barrel GAU-19 Gatling gun (at about 500 pounds.) The dual .50s can put out 18 rounds a second using the older (cheap and abundant) M-2 weapons. Using the new M3M .50 caliber guns a dual gun mount gets you 33 rounds a second, and at more than twice the range of the 7.62mm machine-gun. The GAU-19 can also deliver 33 rounds a second, but in a more accurate and concentrated form than the dual .50 setup. Some ships have been equipped with 25mm cannon, and there has been work done on using the Phalanx anti-missile cannon for anti-small boat defense. But so far, the optimum weapon appears to be the 70 year old .50 caliber machine-gun, either in single or dual gun mounts, or a three barrel Gatling gun mount.
The navy is not talking about what computer simulation, or remote controlled small boat tests were conducted to prompt this increasing interest in mounting more heavy machineguns on their surface ships. But, obviously, a threat is perceived, and these small guns are seen as the solution.