Complex sentence: The M4 Type Carbine is a reproduction of the Colt M4 carbine, but is only in a semi-automatic variant for legality within the U.S. civilian market as per the National Firearms Act.
Split into simpler sentences: The M4 Type Carbine is a variant of the Colt M4 carbine. It is only sold as a semi-automatic for the U.S. civilian market in compliance with the National Firearms Act.