The 338LM 4.050 variant magazine is not compatible with MDT Chassis Systems. The standard length long action magazine in an MDT Chassis is 3.715″ unless otherwise noted in the description stating CIP or 3.85 for the longer magazine well. If you purchased a rifle direct from an OEM or retailer, please check the specifications. Remington Tactical Chassis Rifle and 338 Lapua XCR rifles use a 3.775″” magazine not available from MDT. Tikka T3/T3x and Savage Axis long action calibers in our chassis you must use the 3.560″” option. Will not work with 300 Remington Ultra Mag, 300 Weatherby, and similar cartridges”