My department ordered a bunch of S&W MP15X’s as patrol rifles and we had a few that had issues being sighted in with our current training ammo. They measured the chambers and found that the ones that had issues chamber’s were a little tight.
Apparently S&W had Mossberg make some of their barrels and those are the ones with issues.
Another problem was that the training ammo might be defective also. So are getting new training ammo.
S&W is sending out complete replacement uppers for all of our guns. (I doubt we will get to keep the old uppers, although I would like to keep mine.)
Iowa State Patrol had their Smith and Wesson upper replaced recently, part was the front sight and they added a quadrail. There were some issues but no one has been able to give a difinitive answer to what all the problems were from.
Try to mic, cast, or gauge the barrels before they go back. Also, complete photo doc of the gun and internals would be nice. Add both bits of data to the collective knowledge base.
If ammo is at issue, try it in a known-good. Likewise, get some known-good ammo to try in suspect guns for rule-out.