It's custom. - Adams Arms Standard Picatinny Block Piston Kit

The AR 15 was not designed or intended to run a piston. Which means every AR built with one is custom. A custom build will come with custom problems. Hand guard selection is tricky. Look on the Adams website for a full list of compatible handguards. An easy free float hand guard to slap right in is the Samson Evolution series. They utilize the mil spec barrel nut and are cake to install. Magpul makes a delta ring handguard that fits perfectly with this kit. I built a pistol upper with an Aero Precision Enhanced Gen 2 handguard. However, because the BAR barrel nut is taller than a milspec barrel nut you will have to mill out a recess for the rod bushing to rest in. For anyone utilizing this kit on a pistol length AR I suggest using the kit designed for the 300 AAC. After reading all the negative reviews I can honestly disregard all of them because of either installer error or someone not doing enough research for parts before deciding to build a "custom" firearm. I can't even shoot a GI AR anymore. I can't stand cleaning GI uppers. This AA piston kit delivers on it's promise of running 100% cleaner.