This August, the Marines are expected to take delivery of BAE's (British Aerospace and Marconi Electronic Systems) Remote Guardian System, a GAU-17 7.62-millimeter, mini-gun-equipped, remotely operated weapon that swivels to squirt out 360 degrees of accurate, sustained, suppressive fire "throughout the aircraft's entire flight envelope," according to BAE. This would be a vast improvement over the aircraft's current protection, an M240D 7.62-millimeter gun manned by a crew member hanging on the open rear ramp. But it's a pea shooter compared to the once-contemplated three-barrel, GAU-19 Gatling gun, capable of spraying .50 cals at up to 2,000 armor-piercing rounds per minute (PDF).
The RGS is belly-mounted in a retractable turret, which conserves cabin space and payload capacity. Because it receives input from the aircraft's vehicle management system and continuously computes the impact point, the gunner simply points and shoots, while a computer continually adjusts for wind and vehicle motion.
How quickly the RGS follows the Osprey into the combat zone will depend on how much testing the Marine Corp ends up doing before it commits. BAE tested the system off the back of a Humvee, and it worked for them--twice.