forgive my ignorance, what does regulated for refer to?
Hey I would not have known either a few years ago.... It means to get both barrels to hit close to the same point of aim.
As you can imagine, two barrels would have two different points of impact, and as they are spaced a bit apart, they need to be welded together at the correct angle to make a bullet from each barrel hit in the same spot at a given distance... So when they build these guns they test fire them with a certain round, and then machine and weld until they get them to hit reasonably close at say 50 yards or meters?
I may not have the process described completely accurately but as different ammo and loads will shoot differently in different rifles, so the regulation of the barrels depends upon which
load, not only bullet but also powder, was used to test them with.
So buyers of double guns always want to know this. So they can handload or purchase the ammo that the gun is most accurate with. Often times it is Hornady DGS and/or DGX these days, probably because Hornady makes such a wide variety of the NE cartridges.
However this is a classic bolt gun cartridge so the question is even more interesting, but there is not a definitive answer. and as you can imagine, potential buyer are thinking of the expense they will go through to find a load that does regulate well. At the cost of 416 Rigby, it adds up if you have to shoot 3 or more rounds of each trial load through each barrel! (again, I could be incorrect as to that process as I have never done it)