generally patterning is done with mounting the gun quickly and taking a number of shots that way since this is how shotguns are shot, not aimed like a rifle.
However, for the sake of your test, "aiming" it and finding it to shoot that low at such a close distance does sure indicate that the issue is more with the gun's barrels, boring or choking instead of the stock dimensions on the gun. The point of aim is way off by the sounds of it.
The issue may just be what it is and is not able to easily be fixed. I would speak to skeets gun shop, mike oren or briley about what the options are for re-boring to try and correct.
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B. Dudley
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