What does XM stand for on a Bushmaster AR15?

On a Bushmaster AR-15, "XM" typically refers to a designation for military-style rifles, often indicating that the firearm is designed with features similar to those used by military units. It usually denotes models that are part of a line intended for tactical or defensive use, often incorporating enhancements for performance and reliability. The "XM" designation can also imply that the rifle is suitable for various applications, including sport shooting and home defense.