SW pistols, SD pistols, and M&P pistols all have completely different trigger lockwork. The Original SW(aka Sigma) series was designed to mimic a double action revolver, because S&W thought that's what LE would want, A la Glock. The dang SW was put into production too soon, w/o being Beta tested and proved to be problematic. Then S&W got sued by Glock and Mass. required over the counter pistols to require 10 lb trigger pulls. The end result was a pistol with a long, 10lb trigger pull that no one liked. The SW never recovered it's reputation. As an aside, the newer SWVE series proved to be S&W's most dependable and trouble free pistol in their long history. But the damage was done to the gun's reputation and S&W never could fix it.
The M&P series pistol debuted several years later, w/ a true single action sear and striker action in it. The series was an immediate success, even w/ minor trigger problems that S&W fixed in short order.
Somewhere among the M&P's success, S&W decided to make a "first time gun Owner" pistol that was easy to use, but safe during presentation for 1st time buyers. They went back to the SW blueprints and started over, with the end result being the SD series of pistols. Light, inexpensive, and a trigger that was stiff enough whereas a "newbie" wouldn't slip a shot, but light enough that during real CQB the trigger would preform well.
The secret to both the SW and SD series of pistols is that, with use, the triggers start to feel much better as they break in, w/ the SD being much better than the old SW. Both can be altered to be better, but that can and does lead to problems, both from a dependability standpoint and a legal aspect.
Just my take on the trigger thing. Be safe, all.
