A cheek riser mechanism has a mounting base or saddle that is adapted for mounting to a gun-stock and defines guide rail members. A movable cheek riser member has guide grooves that receive the guide rail members and is linearly movable along the length of the guide rails. Upon rearward movement of the charging handle of an AR15 type rifle the movable cheek riser can be engaged and moved rearwardly by the user's hand or by the charging handle so that it does not interfere with a cartridge charging operation. A riser spring is compressed during rearward movement of the cheek riser and returns the movable cheek riser to its use position upon release of the charging handle.