The Aero Precision AR57 is a highly unique rifle or pistol configuration chambered in 5.7x28mm. Rather than utilizing a standard AR magazine layout, it bridges the AR-15 lower receiver with a specialized monolithic upper receiver group that completely re-engineers how the firearm loads and ejects. VERY GOOD CONDITION NO BOX FOUR 50-ROUND MAGAZINES MAGPUL MBUS SIGHTS STREAMLIGHT TLR-2 HL G LIGHT Caliber: 5.7x28mm FN Capacity: 50 rounds Magazine Feed System: Natively accepts top-mounted FN P90 / PS90 horizontal box magazines Action: Direct blowback semi-automatic (lacks a traditional gas tube, keeping the chamber clean) Barrel Lengths: Typically available in 16.1-inch carbine or 12.0-inch / 6.25-inch short-barreled SBR/pistol configurations Barrel Twist: 1:9" right-hand twist (standard) Ejection Method: Straight downward ejection right out of the open AR-15 lower magwell Receiver Set: Forged 7075-T6 aluminum monobloc construction on the upper; mil-spec Aero Precision lower receiver