Hi Canadian Hunter312,

What is occurring is common with very heavy recoiling ammunition. Your 870 action unlocks after the firing pin drops. And the action is not actually "opening by itself".....it is opening as a result of the momemtum of the recoil. I am assuming you are holding the forearm with your nonfiring hand. When you shoot the heavy recoiling loads your nonfiring hand in an effort to "hold on" to the firearm is actually starting the reloading cycle. (I use to shoot a .35 Whelen in 7600 that was prone to the same thing. The action is NOT "blowing open"......you are actually starting to cycle it unknowingly in response to the recoil. If you watch the entire firing sequence on stop action photography.....it becomes very apparent!) And there is even an advantage......take advantage of the "headstart" to get that action quickly cycled.

Your shotgun is okay! That frequently happens with pump action guns and heavy recoiling loads. Pressures have long since dropped to safe levels by the time the action unlocks! Don't worry about it......and enjoy your shooting sessions!

Dave