The General Dynamics XM312 is an attempt at a very lightweight replacement for the .50 cal M2. With tripod the crew-served weapon weighs 42 lbs compared to 128 lbs for the M2 and is claimed 9x more efficient and accurate. This is achieved through a barrel and gas recoil system that has virtually no kick and allows the shooter to aim through a fixed sight to hit the target with round #1 and sustain hits. It has a low ROF at just 260 rpm compared to 400-500 rpm for the M2 but does not waste all the other rounds that don't score! It can be fed from the left or the right too with disintegrating link ammo. The weapon can also be converted in under 5 minutes into a 25mm weapon through exchange of 4 major parts (which results in the XM307 ACSW). Due to low ROF, however, the XM312 is only fit for the anti-ground role and would be vunerable to helos and low-flying aircraft.

http://upload.wikimedia.org/wikipedia/commons/8/86/XM312-01.jpg http://media.defenseindustrydaily.com/images/ORD_XM312_lg.jpg