The U.S. military has been working since the 1980s to replace the M-16 assault rifle and the M-4 carbine, the main small arms used by U.S. soldiers. The most recent incarnation of the program was called the XM-8 and it was cancelled on Halloween last year based on an audit report criticizing the procurement approach. There was also scuttlebutt that U.S. gunmaker Colt, the holder of the current M4 carbine contract, would have been put out of business if the XM-8 contract and was basically given a reprieve by this delay. Discussion of the decision can be found here, with an emphasis on the argument that the U.S. needs larger caliber small arms than its existing M4 and M16 or the proposed XM-8 (in all cases 5.56mm which is .223 caliber).
It was about then that I lost track of the story. Since then there have been some developments.
A replacement procurement request, also calling for a replacement to the M249 light machine gun has since been issued. The specifications in the new request call for far less reliability and somewhat less accuracy than in the XM-8 program, despite the fact that those standards had already been met in the previous program. Some of the competiting possibilities are noted here, and I suggest that you not stop at the original post, as there are many informative comments.
Meanwhile, a couple of years ago, an entirely separate contract for a new U.S. Special Forces rifle has been awarded to a different design which includes a 7.62mm (.30 caliber) version, for a portion of the order, and this is starting to enter mass production and fielding.
For novices, a carbine is a small automatic rifle intended for use at short ranges, often by troops whose primary duties are to drive vehicles or otherwise act in a support capacity.