AR-57

The AR-57 turns an AR-15 lower receiver into a 5.7×28mm carbine by replacing the upper with a monolithic unit that feeds from an FN P90 magazine mounted flat above the barrel, ejecting spent cases down through the magazine well rather than to the side. It was designed by the small US shop Rhineland Arms and is sold today by AR57 LLC, either as a complete rifle or as a drop-in upper for an existing AR-15/M16 lower, keeping the AR platform’s trigger group and controls while changing everything forward of the receiver. Because ejection runs down the magazine well, an unmodified STANAG magazine can double as a brass catcher when the feed lips, spring and follower are removed.

Where a standard AR-15 upper cycles by tapping propellant gas back into the receiver, the AR-57 cycles by straight blowback, a simpler system suited to the lower pressures of the 5.7×28mm cartridge. It ships as a semi-automatic civilian carbine; a suppressed, fully automatic prototype was shown publicly in 2008 as a proposed personal defence weapon, but the arm sold to civilians has always been semi-automatic only. It uses the same 10-, 30-, 40- and 50-round P90 magazines as FN’s own weapon, giving it a substantially higher standard capacity than most AR-15 pattern rifles.

Every dimension in this entry is drawn from Wikipedia’s citation to AR57 LLC’s own product page for the “AR-57 PDW” carbine. That page survives only as a 2010 archived snapshot of a JavaScript-driven catalogue that does not render its product figures in static HTML, so mass, length and barrel length are recorded as estimated rather than independently confirmed, and no cyclic rate, muzzle velocity or sight radius is published anywhere free. This entry’s one photograph shows a Rhineland-style upper being test-fitted to a third-party lower receiver rather than a production AR57 LLC rifle; no other freely licensed photograph turned up on Wikimedia Commons, Smithsonian Open Access or DVIDS.