Thank you all for the replies. Yes, I am the original owner of my M&P Model 10. I was using new Zero ammo too. I will check again to see if there were any marks left on the bullet cases near the primers.

I have owned many S&W revolvers, and have never had to tighten the bottom screw for the main spring. I guess there is always a "first time" just another "Revolting Development!" Also, I am new to the Forum, and did not know about the special Revolver Section. Thank you.