Unlike other extenders on the market, The Patent Pending design of the MTX permits for a consistent internal wall throughout the extender, which is smaller than the magazine tube. The included spring was engineered specifically for the M&P9 line to allow for unmatched reliability and last round slide hold open. XTech Tactical’s MTX line of extenders are simply unmatched for reliability, ease of installation, and durability! Technical Specifications Custom Engineered Spring Innovative Design For 100% reliable function Tool free, easy installation Rugged Construction Fits Full Size M&P9 and M&P9 2.0 Uses Factory Follower & Body (Not Included) Patent Pending Made in USA