A year ago, the U.S. Marine Corps sent a squadron of 10-12 MV-22 tilt rotor transports to Afghanistan. Some of the MV-22s were equipped with a GAU-2B machine-gun fitted to the bottom of the aircraft. This did not work out as expected, as there were some unexpected problems with motion sickness and weight.
The GAU-2B is a remote control turret using a six-barrel 7.62mm machine-gun. This system has a rate of fire of 3,000 rounds per minute (50 per second), and max range of 1,500 meters. The system weighs 363 kg/800 pounds, including 4,000 rounds of ammo. A member of the crew uses a video game like interface to operate the gun. That's where the problems began. It seems that, while remote control turrets work fine in ground vehicles, even while the vehicle is moving, such is not the case in aircraft. There, the movement is all over the place, and while staring at the remote control turret gun sight screen, the operator tends to get motion sickness.
The other problem is heat and weight. Normally, the MV-22 has a lifting capacity of about 5.5 tons, of which the GAU-2B comprises 6.6 percent. But in hot weather, the lifting capacity of the vertical takeoff MV-22 is cut about a third, to 3.7 tons. Suddenly, the gun turret is taking up 13 percent of your payload. Because of these two problems, the turrets are mounted on the MV-22s only when it is pretty certain that such a weapon is very likely to be needed (which is not much of the time.)
The GAU-2B is part of the Universal Turret System (UTS) for Helicopters. Plans for arming the V-22 have been discussed for nearly a decade. The original proposal was for a UTS equipped with a 12.7mm machine-gun. That has a longer range (about 2,000 meters, compared to 800), but the 7.62mm GAU-2B could lay down more bullets more quickly. Experience in Iraq and Afghanistan indicated this would be a more useful defensive measure. The UTS will be mounted on MV-22s (and SOCOM CV-22s) as needed. The machine-gun turret is mainly there for protection from local threats, not for turning the V-22 into an assault aircraft. The marines also plan to mount an M-2 12.7mm machine-gun on the rear ramp as well. Currently they sometimes use a 7.62mm machine-gun there, but the 12.7mm weapon has been successfully used on the rear ramp of large helicopters. There is no motion sickness with weapons mounted on the rear ramp, and they are much lighter than the remote control turret (which, of course, have the advantage of being able to swing around 360 degrees.) Thus you have another case of old and simple being superior to new and high-tech.
The marine MV-22s can carry 24 troops 700 kilometers (vertical take-off, level flight, landing, and return) at 400 kilometers an hour. The MV-22 is replacing the CH-46E helicopter, which can carry 12 troops 350 kilometers at a speed of 200 kilometers an hour. The MV-22 can carry a 4.5 ton external sling load 135 kilometers, while the CH-46E can carry 1.3 tons only 90 kilometers.