A non-firing housing imitates a functioning receiver for an M249 machine gun. The non-firing housing includes one or more of a number of alterations as compared to a functioning M249 receiver. Such alterations include: providing a non-firing barrel support with substantial dimensional differences as compared to a standard M249 barrel support; providing a non-firing barrel support with additional structural features that prevent any bolt head (and particularly a standard M249 bolt head) from entering the battery position; providing right or left non-firing bolt rails with dimensional or structural or positional alterations as compared to standard right and left bolt rails for a standard M249; providing the non-firing housing with the absence of an ejection port; providing a non-firing right bolt rail with the absence of a hinge bore; and providing a non-firing left bolt rail with the absence of an ejector pin bore and back stop material.