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”