Much of the rest of the world's knowledgeable doublegun builders (shotgun and rifle) and smiths use the term "regulation" to refer to where the individual barrels deliver their payload in relation to each other. What is strived for is that both barrels shoot to the same point of impact at a certain distance. gunman, in his above post, seems not to ascribe to this, preferring to relate regulation to pattern percentages at a certain distance. A simple search of the internet for "barrel regulation" will provide an abundance of evidence, and information, about how barrels are regulated to shoot to the same point of impact. Double rifles are much more finicky about this, most having to be used with a particular load in order to deliver bullets to the same POI.
If a shotgun is regulated to shoot both barrels to the same point at, say 30 yards, when manufactured by the maker, it will usually continue to do so with the chokes opened up, providing that the smith who does the work is knowledgable enough to pilot the choke reamer with the bore axis. Nonetheless, he should check for regulation before and after the work is done, as a matter of professionalism. There is a chance that the barrels were not properly regulated when he received the gun, and some will test fire it before doing the work to see what they are working with. Sadly, many gunsmiths don't care about such critical details. Better doublegun smiths do care, tho'. There are several who post on here that have very high standards for their work.
SRH
That's also how I interpret regulation. Makes little difference what percentages your gun throws if the patterns don't go where they're supposed to. Patterning a shotgun for point of impact (POI) is actually a 2 step process. First, you have to make sure the gun shoots where it's supposed to, which you do by aiming it (some people do so from a rest of some sort) to determine that the pattern is centered on your aim point. If the barrels are "properly regulated", the patterns will hit where you're aiming the gun. The second step is to make sure that the gun fits the shooter. If it doesn't, the pattern won't end up where it's supposed to, even if the barrels are properly regulated. (To paraphrase OJ's lawyer, when you're shooting at a moving target: "If the gun don't fit, you will not hit.") That involves mounting and firing without "aiming", but rather pointing the gun as you would at a clay target or bird in flight. You may then have to make stock adjustments--drop or cast--to make sure that the gun's fit is also properly "regulated" so that the patterns end up where you're looking when you mount and fire at a moving target.
Getting the appropriate % of shot in a pattern at 40 yards is a question of choke regulation, not barrel regulation. As Stan points out, if the gunsmith doesn't mess things up, opening choke shouldn't impact barrel regulation. The pattern should stay in the same place, but with fewer pellet strikes in the 30" circle.
