With or without the safety, the trigger action to striker release is the same in both M&P models. The safety just prevents the trigger from moving. Just the way S&W wants to describe the action. Maybe S&W feels the terminology of "double action" sounds safer to some people.

For all practical purposes the M&P's are single action. But S&W calls them double action (see the reason below). Understand one action and the hammer/striker drops while double action requires two actions for the hammer/striker to drop. As explained in a prior post most revolvers can be both.

A single action revolver is when the hammer has been manually cocked. By pulling the trigger, the hammer is dropped. This only required one action to drop the hammer, just pulling the trigger. Normally in a single action revolver trigger pull, the trigger pull length is very short. A double action in a revolver is when the hammer is down. The trigger pull causes the hammer to move back, and normally causes the cylinder to rotate. By continuing pulling the trigger back, the hammer will drop. This is double action because the first action of pulling the trigger caused the hammer to move back, and the further pulling of the trigger caused the second action, the hammer drop. The double action trigger pull length is normally much longer than the single action trigger pull.

In a single action pistol, the trigger pull causes the hammer/striker to fall. However why S&W calls this double action is due to the fact the sear on all M&P pistols, has a slight raised "bump" on the sear face. With the trigger pull, as the sear moves down, the bump contacts the striker and the bump pushes the striker rearward about 1/32 inch. Since the trigger pull is doing two actions, moving the striker rearward (1/32 inch) and causing the striker to release (drop), this is two actions or double action. It is a very trivial reason to call it double action, for 1/32 inch of striker movement, but it is double action.

A true double action pistol would be similar to my Ruger LC9 and many other double action pistols. The trigger pull causes the hammer to move back (cocking the hammer) and by continuing to pull the trigger the hammer will drop.

My Shield 40 had the bump on the sear. But when I replaced the sear with an Apex sear, which does not have the bump on the sear, I made my Shield go from double action to single action. Not really as the M&P's are actually single action (sorry S&W but it is not a true double action), with a tiny bit of striker rearward movement.

Bob