Trouble with 870 opening after shot?

[plinker 777]

^ an 870 shouldn't do this. I have a few wingmasters and the only way the action will move is if I move it. (intentionally or unintentionally)

 

[aaron v]

Do you have a mag extention? If so, is the barrel nut tightened before the extention is tightened? I had a 20 g that would open itself with even target loads and that was the problem.