The 6400C is the civilian (post ban) version of the M4. It has no bayo lug or flash hider and has a pinned fixed fake collapsible stock. If any of these are important to YOU I'd put the $900 towards a LE6920 which is the "legalized" LEO M4 sold previously only to Law Enforcement. Now they have all the "EVIL" features in a brand new Colt, since the ban has sunset. OR you could always buy a 6400C and add a M4, Vltor or Magpul collapsible stock, flash hider etc.... on your own if you wanted.