The part number of the spring that flies off when you remove the rear sight from any M&P is 27449 and is available from Smith. The part is the same for all the current full size and compact models. If you don;t already know, this problem can be avoided if you know the spring is there and capture it while slowly drifting out the rear sight. There is a round silver cap that covers the spring. Just apply a little tension on this cap until the rear sight is clear of it, then slowly release pressure and hold on to the spring. If you loose it, call and order the spring. Or do what I did, order a few. They are under 2 dollars.