I have noticed on several guns and even since I was a kid that on some of my 880s the receivers won't stay closed. After a bit more thought I think it is aggravated by scope mounts (examples below). The dovetail is on the right receiver and when the scope mount clamps down I think it is putting pressure on the receiver halves and pushing them apart. The receiver halves only connect directly through the rear large stock bolt. They are cantilevered off the main frame. I think this could lead to the Zero drifting over time.
The new plastic receivers have two additional screws that connect the halves near the dovetail. I think adding similar connections for the metal receivers would help accuracy and prevent the zero from shifting over time. I found some 3mm aluminum standoffs that looked like they could work. I took a salvaged receiver and drilled a 1/8" hole above the bolt guide on the inside of the right receiver. I then put the two halves together and drilled through the left receiver using the previous hole as a guide. I did the same thing to the front of the receivers making sure the first hole was above the barrel. I then used 3mm pan head socket screws to connect the halves. I screwed one side into the standoffs then put the halves together and installed the screws on the other side. The result was a nice and securely assembled receiver set. I will be doing this mod to all my modified guns with metal receivers.
I have added this mod. to three of my metal receiver guns as of writing this blog and can say with certainty that it as improved accuracy from shot to shot as well as over time. Below is a video this more info and applicability to the 822.