Like NAK posted, I have an M&P that was build in early May 2012 that was not accurate... actually would not group out at 15-25 yards. I would have shots off the paper (25yd slow fire bullseye target) at 25 yds. This was sandbaged on a sturdy bench. Sent it to S&W and they replaced the barrel. My gun was in their shop the same time as NAK. One trip to the range and I'm on the paper getting decent groups of 4-5 incheds at 25 yds... with the still occasional flyer out of nowhere. For a defensive gun, it is now accurate enough. For a competition gun, I shoot better than the gun can. I have a new Glock 34 that was accurate right out of the box. I like the feel of the M&P much better (apex competition AEK), but the glock is more accurate. The glock with a .25 cent trigger job and competition springs is still not the trigger on the M&P.
I will probably try the #20 spring like NAK did to see if I get further improvement. Certainly, the new locking block that apex is developing should increase the dwel and improve the early unlocking issue this gun has.

My new barrel is definitely a tighter fit in the slide and locking block. For those non believers out there... well, my targets tell the truth. Still can't explain the occasional flyer... that is the weird part.... and these flyers are off a sandbag. At least now for the most part I can call my shoots with the M&P.

Believe what you want, but the accuracy of these FS 9mm varies from gun to gun.

Cw