Polishing the entire outer surface of the plunger reduces grit. I don't reshape them, just break the sharp edge. Some of the gritty feel is the trigger bar sliding under it- but some of the grit is from the plunger moving in the slide. I polish the whole outer surface of the plunger, by chucking the plunger directly into a drill motor and spinning it against 400, 600, and 1000 grit paper. I also chuck the spring into the drill and polish the outer surface of the spring, as it moves in and out of a hole in the plunger, and if rough, can contribute grit. Then I polish the hole in the slide the plunger moves in, using sandpaper rolled up and chucked into the drill, then polish with a Q-tip with cotton wadding wound around it. This also works in the firing pin spring channel.
Yes to the M&P also! I did this to my M&P9C after looking at the prices of the Apex stuff, I just did the $.25 Glock trigger job to the M&P, and it smoothed it out a lot.
Taste the wares, Email.