The Challenge To The M4

Military.com:

Army Agrees to M4 Sand Test Shoot-Off

Finally:

After months of heated debate, the Army will conduct a side-by-side test shoot next month with its standard-issued carbine to see how well it can withstand extreme dust and sand environments.

The Competition

The shoot off will test the capabilities of the M4/M16 operating system against three other rifles: the Heckler and Koch-built HK416, the FNH USA-designed Mk16 SOCOM Combat Assault Rifle and the previously-shelved, H&K-manufactured XM8 carbine.

All three competitors use a gas-piston operating system that requires less maintenance and has demonstrated in some tests that it can fire accurately even if completely fouled with sand, dust and mud.

More on the ongoing debate surrounding the M4, 416, and others here: Reconsidering the M4.