I've been reading this forum and just joined today.
I have owned the M&P Sport since Oct of 2011 and after firing approx' 300 rds. of Lake City, Freedom Munitions and Remington I could not pull the bolt back more than 1/2" using the charging handle. I had to remove the rear pin and seperate the lower from the upper half and refasten which allowed me to fire a few more rounds and the same thing happened.
I sent it back to S&W and so far no problems.
I just wish they would state on the repair invoice the problem or at least what they did to repair, not even a 3 digit code that most companies use to distinguish the problem for tracking purposes. So far the Sport has worked fine and I am in NO WAY bashing S&W. They make great products and they stand by their warranty.
If any one knows what the problem could have been please share.