ABOUT COLT 6721 UPPER FOR SALE

  The magwell engraving has transformed, and Colt has rebranded all their carbines as "Carbine" and not marked M4 Carbine.  This is the only change towards the solution. Honestly, I wouldn't shoot just one.  Time to move on to a fresh rifle.  Get an LMT or HK if you like army design and style pistons.    Colt has no real interest in helping