most are 5.56 or 7.62 but seen a .50 BMG and even a 20mm on a Navy R2D2(CIWS) hear there is a caseless round version in developement for the Army to be mounted on the bradley as a CIWS(close in weapons system) to shoot at Rpg's but they are having a problem with targeting of friendlys so they need to adjust it to detect incoming and distinquish it from outgoing.(totally automated just flip the safety off)
By the way the jesse ventura one in predator was pure

The first was that the Minigun weighed in at thirty pounds, which was heavy, but carryable. A backpack with a thousand rounds of ammunition and a linkless belt to the Minigun weighed in at another thirtyfive pounds. Backpacks with two thousand rounds weighed in at sixtyfive pounds, and were totally unrealistic in size. Just the gun and the ammo weighed in at sixtyfive pounds.
The second was that in the movie, the power for the Minigun had been supplied by a pair of truck batteries through a cable that simply ran over the ground, and up into the Minigun. Since those batteries weighed some 30 pounds each, it was obvious that only one could be carried by one person (in addition to all the other equipment the soldier was to carry). Thus, the weapon could never be fired at its full rate of fire.
The third problem was that even at "only" a 1000 rpm rate of fire the gun produces about 11 kg of recoil *continuesly*! And this amount of force increases geometrically in proportion to the rate of fire. Firing a large burst would result in the gunner being spun around by his own weapon, and spraying everything around him indiscriminately with bullets. Including his own comrades.....
The XM214 Automatic Gun (aka the Minigun) was developed for use mounted in and on helicopters and light aircraft. Like most G.E. Gatling gun type weapons it has six rotating barrels and the potential for a absolutely incredibly high rate of fire. It is electrically driven, and has a firing rate that can be adjusted from 1000 rpm all the way up to a unbelievable 10,000 rpm. In addition to that, it can be set to fire bursts from 30 to 1000 rounds. A real drawback to the higher rates of fire is off course the huge ammunition usage (166 shots per second) , and the power requirements, because firing it at full power it requires some 3.2 hp to drive the barrel assembly.