To answer ray f's question:

There is no difference internally between the two firearms (other than the presence of the safety selected switch). If you picture a revolver- you can either puller the hammer back manually then pull the trigger to fire the weapon (single action), or you can simply pull the trigger to fire the weapon (double action). As with the M&P, there is no hammer to pull back; therefore simply pulling the trigger is the only way to fire the weapon. This is my understanding of the whole single action double action thing. I could be way wrong. Hope this helps.