A complete upper pairs barrel, receiver, handguard, and (on most models) bolt carrier group into a drop-on assembly — push two pins and your lower has a new caliber. Chamberings run 5.56 NATO and 223 Wylde through 300 Blackout, 6mm ARC, 350 Legend, and 308 Win. Shop precision billet uppers from Radian and Battle Arms, long-stroke piston MK1 uppers from PWS, and duty-proven builds from Daniel Defense, Colt, and Sons of Liberty Gun Works.