The ArmyTimes has written about how the army is stuck with the M16/M4 while spec ops are moving to the H&K 416:

Delta Force worked with a gun maker to come up with a better weapon. The 416 is now considered in many circles to be the best carbine in the world, but the regular Army is sticking with the M4 and M16.

Members of the elite unit linked up with German arms maker Heckler & Koch, which replaced the M4’s gas system with one that experts say significantly reduces malfunctions while increasing parts life. After exhaustive tests with the help of Delta, the H&K 416 was ready in 2004.

Members of the elite commando unit – formally known as 1st Special Forces Operational Detachment-Delta – have been carrying it in combat ever since.

The 416 is now considered in many circles to be the best carbine in the world – a weapon that combines the solid handling, accuracy and familiarity of the M4 with the famed dependability of the rugged AK47.

For the foreseeable future, however, the Army is sticking with the M4 and M16 for regular forces.

More here.