3 M&P's of different vintage with the same accuracy problem; barrels shot out with only 5-8K rounds. Not saying it can't happen, or maybe there is a "fit" problem with the barrels. The M&P series isn't a "target" grade pistol, although if you spend enough money they can come quite accurate. That's not something I chase in a service grade pistol.

I have 3 FS M&P's-the 9mm being the only NIB manufactured in 2013. My FS M&P 45 is a 2007 and my M&P 40 is a 2010 Police Trade-In. They will all shoot 2"-3" groups at 10 yards off hand with any ammo I feed them-factory or reloads; lead, plated or jacketed. Off a rest, I could probably get better results. I'm not near as good with my rifles, never been very good with long guns.

You've had several offers to re-coup your money, I'd even take one off your hands and shoot it, before I ever spent a dime for a replacement barrel. Then if it didn't shoot to my standards, I would opt for a new S&W replacement. IMO, a good, clean used pistol should shoot 2"-3" groups (minimum) at 10 yards, but that's just MY experience.....