At the end of the day, Marines are Marines. They have weapons, ammunition, equipment, a fighting unconqueable spirit, (with the exception of Muhammad Aidid and a couple others) an unbreakable loyalty to their nation, and in my friend Rich's case, an IQ of 12. Haha just kidding, I love and respect the Marines!
For your UNSC Marine, build a Marine, put a Halo helmet and armor on him, give him a halo weapon, and some 405th patches of your choosing, and viola! A real life halo Marine.
(We're ignoring the halo writers who turned many of them into cowards)