The GAU-8 "Avenger" is used on the A-10 Thunderbolt II (Warthog). A 30x173mm cartridge in High Explosive Incendiary, Armor Piercing Incendiary and Depleted Uranium. Rate of fire is 3900 rpm.
The GAU-19 is a 3 or 6-barreled gun chambered in .50 BMG (12.7 x99mm) for use on vehicles and helicopters. Rate of fireis 1000-2000 rpm 3 bbl. and 4000-8000 rpm 6 bbl.
The newest is the XM-556 developed by Empty Shell Defense, LLC. It is an extremely light (16 pounds) and small (22" long,6" high and 9" wide) gun designed to be carried by one person. Rate of fire is 2000-4000 rpm. More information can be found at: emptyshell.us NOTE: NOT available for private ownership, governmental agencies, military and "certain others" only. Sucks!