So you're thinking it's unlocking the chamber/forearm as the slug is still in the barrel, causing slight movement thats affecting accuracy?
In my opinion, I don't see this being the problem. Afterall in an autoloading shotgun the shell gets ejected and their is a heck of a lot of movement in the action as soon as one pulls the trigger. If that was the problem, then autoloaders would never be able to hold a group.