Many of the current ground weapons used by US Soldiers have been in use for decades. The M2 .50 caliber machine gun has been in service since 1933 and the M16 family of weapons have been in service since 1963. Weapons that have a long service life do so because of successful design and function. Over the years many weapons aimed at the military have come and gone. Often times these new weapons don't offer much of an improvement over currently used weapons to warrant a complete change over or they fill a niche that is not really necessary.
First is the XM806:
XM806 .50 Caliber Machine Gun
This .50 caliber machine gun is looking to dethrone the current king, Moses Browning's M2. The XM806 is suppose to be more accurate and half the weight of the M2 and some rumors say that it will not need to be head spaced and timed before operation. Read more at Kit UP! Blog.
Then there is the XM25:
The XM25 has just entered limited use in Afghanistan. This is a revolutionary weapon due to the fact that it is designed to target enemy targets behind cover. Not in the traditional way of shooting through the cover, but instead detonating a round behind the cover. They are calling it a 'Counter Defilade' weapon. "The bullet has a small magnet inside that lets it generate AC current as it spins and a microprocessor that measures those current oscillations to derive how far it’s traveled.” To read more check out The Truth About Guns blog. I fear this weapon in its current configuration is doomed to fail, because of its size. A Soldier will have to carry this weapon in addition to their primary weapon. I feel that once scaled down this may be a viable weapon to replace the barrel mounted M203 grenade launcher.