This is a brand new Panzer Arms AR57 ULT Upper Receiver chambered for 5.7x28MM.  This upper receiver features a steel 16" barrel with 1:8.5" twist and a 1/2x28 thread pattern. This AR57 Upper is constructed with a steel barrel, muzzle brake, bolt carrier group, buffer, and screws, all built into an alloy monobloc receiver. The 3 slot MLOK rails will allow you to add your favorite light or optic making this one tough tactical upper. Convert your AR-15 Rifle to shoot the same 5.7x28 ammo as the legendary FN P-90 PDW as well as use P-90 magazines. Your standard AR magwell becomes the ejection port with this upper. Includes a box and one magazine. Extra mags can be purchased as long as capacity is legal in your state.