Complex sentence: The winner of the Individual Carbine competition was to supplement the M4 carbine in U.S. Army service; all other services will to continue using the M4 carbine and M16 rifles.
Split into simpler sentences: The winner of the Individual Carbine competition was to supplement the M4 carbine in U.S. Army service. This was for the U.S. Army only while all other services will continue to use the M4 carbine and M16 rifles.