I just got a remington 870 mag 12ga. It is used but like new. It will pump easy and smooth until you put a live round in it and then it will chamber hard and eject hard. The shell will slide in and out of the chamber easily with the barrel off, so the chamber is not the problem and it does this with three different loads/manufacturers. It feels like the shell is causing the bolt to bind up or something. I have never had this problem with my nova, BPS, 37, or 410 870.

Has anyone else had this problem?

Thanks, for your help

Sounds like you might have a bent carrier or other part. Take it to your local smith and he'll get it going in a flash.