The M4E1 Superlative Arms Complete Upper Receiver Groups, built by Aero Precision, feature the well-regarded Superlative Arms Piston Kit. This Upper Receiver was spec'ed out by Brownells and Aero Precision to integrate seamlessly with the Superlative Arms Kit. Perfect for a Suppressor host or for those who simply don't like cleaning the chamber area of your AR-15, these Upper Receivers vent hot gasses and debris at the gas block, not in the receiver like a traditional gas impingement AR. The adjustable Superlative Arms Gas Block allows you to fine tune your gas setting and can be easily adjusted through the front of the handguard. The gas piston can eas Mfg: Aero Precision