Terrible S&W factory barrels for M&P . . . .
There is a well known problem with the S&W M&P barrel. They unlock too soon, and that enlarges 25 yard groups to 5" or more with MOST of the M&Ps.
I have (2) M&Ps. Both are .40 cal. and they rarely beat that level of performance. I have installed several APEX barrels in 9mm M&Ps they all now shoot consistent 1" groups off sandbags with the Vortex Venom reflex sight and the APEX trigger.
So . . . . I recently installed a .40 cal. Bar-Sto barrel (because APEX doesn't currently make a .40 cal. barrel). This Bar-Sto barrel now helps the M&P shoot 1" groups at 25 yards . . . .
HOWEVER . . . . The extreme amount of time to fit a Bar-Sto barrel makes this a massive project, and it occasionally fails to fully lock in battery with perfectly good dummy rounds. I've found that the Bar-Sto "Match" barrel comes without several machine operations, and the fitting job is twenty times more difficult than fitting any APEX barrel. (Bar-Sto charges $175. to fit one of their own M&P barrels).
This .40 cal. M&P with the Bar-Sto barrel cycles perfectly without ammo. It feeds great with ammo, and it also extracts fine . . . . but it intermittently fails to fully lock in battery with a "known good round" in the chamber.
This is a mystery . . . . I've heard this is a common occurrence with the Bar-Sto barrel. I've tried loosening up the barrel fit .001" at a time (more than what allowed it to function without ammo). Let me know if anyone has a fix for this.
(I don't need any guesses.)