Thanks Brian - good point. I did consider that and you are clearly right. In my case it wouldn't shift enough to be problematic but that would have to be considered on a case x case basis. I wouldn't want to go more than about .02 off center, personally. The area I would want to study closely and understand clearly before adjusting flats would be the fore iron.

I have to say, I'm really glad that Tom asked his question because I got the chance to learn that I don't know what it is I think I know about double gun design. I've got some studying to do before so much as putting a file to any working surface. The barrel face is nested in the breech face - that sends forces somewhere, and it's not how I visualized it, at all. Heck, I'm feeling uneasy about a simple shim at this point. I can gather and cipher some numbers and all but, can anyone point me to some legitimate working / manufacturing drawings of a box-lock action? Reference materials that will short-circuit the learning curve?

I had Brad Bachelder fit up a set of Parker Repro barrels Q1/Q2 to a GHE and we decided that as long as the face of the breech was tight to the frame we'd just forgo working down the top of the breech balls and re-engraving the top rib. I shoot it and while it's a a little proud, I do well on skeet with it and look forward to taking it to northern Maine this season on grouse and woodcock. Maybe sometime down the road when I have some extra cash on hand I'll have Brad finish it off. My advice is as long as it's tight to the frame and it doesn't bother you too much, let it be for now and just shoot it.