Sig Sauer MCX Receiver Wedge. This soft, black rubber wedge keeps tension on the upper and lower receiver to facilitate a tight fit. The part will fit Sig Sauer MCX rifles chambered in .223 Remington, 5.56 NATO and .300 Blackout. Made to the highest Sig Sauer standards in material and workmanship. This factory original part is perfect to replace your original worn parts, or to just keep your Sig Sauer firearm running in top operating condition. From the world leader in firearms, genuine Sig Sauer parts are designed around the same relentless standards put into every pistol and rifle they make. Kit up, dress up, or replace your original parts with nothing but genuine parts from Sig Sauer.