From The Manufacturer/Wikipedia/People Smarter Than Us:
Each new rifle in the M.A.R.C.901 family is an evolution of the LE901-16S, which means the upper receiver group can be easily swapped out for any Colt upper receiver chambered in .223 Rem (5.56 x45 NATO) with the conversion kit (sold separately). Bolt and magazine controls are ambidextrous for flexibility and ease of use.