The reason for the SD ammo being required for the SD weapon is due to the fact that the gun fires silent. While yes the real gun can and will fire standard ammunition it doesn't do that silent. Two things make the gun go bang, the explosion from the gunpowder causing gasses to expand and the bullet going supersonic. The silencer built into the MP5SD will remove the first bang, the expanded gasses go into the silencer allowing them to cool and not "bang" when the bullet exits the barrel. However, the silencer has no control over the speed of the bullet so it would still go bang since the bullet would go supersonic if it is standard ammunition. Now unless ARMA already has this programmed, that means they would have to make every SD weapon play sounds dependent on what ammo is equipped and not the weapon itself. Seems like they did a good job to me, since if I had an SD weapon I would want it silent. Though depending on whether or not they can program this is and how long it would take, it would be nice to shoot standard ammo out of the gun if that is all you had in a pinch.