ricklee:
The silver striker is the one to have.... +1 there
....
The replacement sear improves the trigger, but isn't otherwise necessary. However, with the replacement sear, and in some cases, without it, you can run into the dreaded "dead trigger" problem....
Essentially (seems to be some combination of dirt and sloppy tolerances), the sear doesn't move into the right place to grab the striker during recoil, and the gun won't fire. There's no "cocked" indicator, so you really don't know unless you are quite into the feel of the trigger, and I wouldn't trust that.
If you have the problem, S&W will fix it.... But it's probably wise to schedule a replacement long before that, and S&W isn't being a lot of help there. It's quite easy to swap the whole mess (you can get replacement sear blocks from Brownells and Midway), but nobody's saying much about which version they're stocking/shipping at this point. You need a good mill and a machinist to do this, unfortunately....
The good news is that it's inexpensive, and you can remove the sear block and mail that any old way (Apex likes FedEx or UPS for tracking) 'cause the sear block is not considered a firearm, and doesn't need any of the expensive shipping magic. Turn around was pretty quick, too....
(I have three M&P's. One needed the fix as soon as I put the DCAEK kit in it. The other two are fine. They're on the list - one of these days....)
This seems to be the same for all of the smaller caliber M&P's - 9mm, .357 Sig, and .40. They use the same innards, although the 9mm slide's breech face is a tad smaller. You can buy a 9mm barrel and magazines from somebody and put 'em into the .40 like the gun came that way.... Aftermarket 9mm barrel, though - they appear to be made to fit the .40 slide. You can't put a .40 barrel on a 9mm slide, though.
The .45 versions are slightly different, and less interchangeable, but the dead trigger seems to turn up there, too. The fix, in all cases, is a somewhat larger spring and plunger to return the sear to it's proper place to grab the striker during recoil.
Search on "Dead Trigger" and you should find more.... Seems like some guys will report "never", and some will report (as I did) "it happened before the 'new' wore off."
S&W is slipstreaming the new sear blocks into production, but unless you get a MA-compliant gun, it's still a coin toss....
Regards,