smith and wesson m&p 9mm compact with thumb safety
I bought an M&P40 last December and I have a suggestion for you to smooth out your trigger and lighten it a little at no cost and no disassembly.
Remove the magazine and make sure it is void of ammunition. Then check the gun and also make sure it is unloaded - do this again to absolutely be certain. Install the magazine, pull the slide back a little to set the trigger, then pull the trigger.. as in a dry fire. Now just start pulling (working) the trigger without dry firing. In other words, do not work the slide each time before pulling the trigger. Do this many, many times. I've done this to mine over 6000 times. You'll be surprised at the difference in you trigger.
Oh yes. From time to time, do dry fire it to test your progress.