nialmac,
I may well be contributing to the language barrier. If I am, I am sorry. But, in an earlier post on this subject I said that "I can make them shoot to where I am looking if the barrels are regulated to each other, and if the required adjustments are within reason". I assumed that anyone with enough interest in this subject would read ALL posts on it, and remember them. I didn't feel the need to repeat it all again. If you take exception with my way of stating it maybe you need to ask yourself if a gun that shoots "20 feet away from where the gun is pointing" is, as I put it, within reason.
Your example is certainly UNreasonable. I have fitted shotguns to myself that did not print where I looked, and some were off considerable. But, none more than maybe 15" at 30 yards. I know you used an extreme example to make a point, but doesn't my earlier post cover what you asked?
Again, I apologize if I contributed to the language barrier. Maybe this helped to clear it up, or not?

Stan