5.56 NATO, 6.8 SPC, 6.5 Grendel, 6mm ARC (new 2022), .224 Valkyrie, 5.45x39mm (disc.), .300 AAC Blackout (new 2015), or 7.62x39mm cal., GPO, 14.7, 16, or 18.6 in. light or heavy profile chrome-lined barrel, GB2 or GB3 adj. gas block, A2 flash hider, MB2 or MB3 muzzle brake (muzzle device permanently attached on 14.7 in. barrel variants), uses AR-15 mags., aluminum receivers, two-stage trigger, folding BUIS, side folding stock (standard until 2008, stock configuration optional beginning 2009), FAST2, or FAST3 folding stock (currently available), Picatinny rail handguard (disc.), KeyMod, or M-LOK handguard, ambi controls, left-side charging handle, black, FDE, or OD finish, 7 1/2 lbs. New 2006.