The Faxon Firearms ARAK-21 is a long stroke piston system with a quick change barrel and a monolithic 6061-T6 aluminum billet upper, available as a complete rifle or complete upper that will fit onto your existing lower. Faxon lowers are now machined out of weight saving magnesium. Other features include: adjustable gas regulator, non-reciprocating foldable charging handle, bolt carrier rides on hardened steel inserts in the upper, improved 8 lug bolt with dual ejectors, nitride finished 4150 barrels.