You might want to try 30-40 yards. This will give you a better idea on the barrel regulation. If still bad, send it back to Stoeger.
If it is off at 15-20 yards it will off even more at longer distances. There is no "English" causing the shot charge to curve back in. Patterns with the 410 are traditionally shot at 20 yards due to the small shot charge. At 30 and especially 40 yards the patterns may be sparse enough to have difficulty determining where the center even
I've heard of guys fixing POI issues on cheap doubles with round files and careful chamfering(offset chamfer). I've a friend that zeroed his turkey gun in this manner on a cheap Yildz. It's worth researching if Stoeger doesn't replace it with bbls that have proper POI.
I hadn't heard of that with shotguns but that is what was attempted to fix the barrel regulation problem on the cheap $5000 double rifles sold through Cabela's a couple decades ago. It didn't work too well. I would suspect the process outdoor also open up the choke some which may not be desireable with a 410. It would probably be easier to send the shotgun to Briley to see if they could thread it for their non-concentric choke tubes though I believe that would be about the price of the Stoeger if not more.
