Ive as well have an older 870, turkey loads and also steel shot will do the same thing you are talking about. Dont qoute me, but everytime you shoot, the guns action is released for opening and ejection of the shell. With the higher pressures, your action is helped along its way and pushes slightly back, instead of you having to start that action yourself. I have never had any problem with that. Of course the first few times it happened, I was worried. Just in case there is something going on, I would still check with a gun smith. Good luck