The MX SW fires from the closed bolt position, just like the standard versions of the mx do. However, it's more appropriate for it to fire from the open bolt position, to prevent cookoffs, which are very likely in suppressive fire weapons. Note, it is reasonable for a lmg version of an assault rifle that fires from closed bolt to fire from open bolt, and in real life there is a derivative of the M16 which fires from either open or closed bolt, depending on it's semi/auto setting.