Jeff

Generally a gunmaker will take some commercially available ammo thought suitable, rightfully or wrongly, and regulate the barrels for it.

If you are interested in how regulation is done, there are some posts, probably in the building double rifle forums where it is described. But basically the barrels are not fixed in place, rounds are shot through both barrels, a wedge or similar moved, until the barrels are regulated/shooting to the same point of aim at a given distance. Then they are soldered/fixed in place, ribs put in etc. Alex Beer re-regulated his own rifle described in a recent post after adding a scope to it.

This is one of the processes that adds a lot to the cost of a double rifle. One reason you will find cheaper doubles are not regulated at all and use other less reliable methods, or use "user" operated systems where the barrels aren't fixed at all.

BTW as your Gold Sable is an Under and Over, the same principles apply to it as a side by side. My notes above regarding Left and Right, just become Upper and Lower barrels.