Unlike other extenders on the market, The Patent Pending design 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&P Shield 9mm 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 Fits all 9mm M&P Shields Adds 2 rounds to the factory 7 and 8 round magazines Uses factory 8rd sleeve when used with the 8rd magazine Tool free, easy installation Custom engineered spring Optimal ergonomics and texture for recoil control Uses factory follower & body (Not Included) Patent Pending Made in USA