Seekins Precisions Billet IRMT-3 Upper Receiver uses Integrated Rail-Mounting technology (IRMT) to deliver the rigidity of a monolithic upper while maintaining the convenience of modular rails. Wide design enhances your accuracy. Externally threaded steel barrel nut with 1-1/8 wrench flats offers easy installation with standard tools. Factory-installed forward assist and dust door. Compatible only with the Seekins Precision SP3R V3 rail system. Made in USA. Type: AR-15 Uppers.