The problem, when using existing shotgun monoblock, having one gradual taper from chambers to muzzle end, is that it forces one to use rifle barrels that are tapered too quickly, losing needed weight that should be concentrated as close as possible to the action; between the hands when in shooting position.

As I tried to explain briefly, in one of my earlier posts, and is so well explained by Rhodes above,
Most better double rifles, especially British ones (yes, my friends, the early day British double rifle maker knew what he was doing), had the barrels close to parallel for 3 inches or so
toward the chamber end (I suggested 6 inches or so, since you are using a shotgun action, which is lighter in weight than double rifle action----of course, to do this properly, one would have to make from scratch, and use, a custom monoblock with no taper (that is all explained in Brown's book), thus allowing barrels with no taper out to 4 to 6 inches or so, with gradual taper from that point, out to another 4-5 inches, finally after that, a gradual taper to muzzle. This is all done to concentrate weight around the action, between the hands, when in firing position. Making a new monoblock from scratch takes more time, but it is well worth it when you are doing a .500 nitro conversion from 12 gauge shotgun, since end result is a double rifle of proper balanced weight.
Most guys make their double rifle muzzle diameters too large, too heavy, due to fact that they have become so accustomed to making barrels for bolt action rifles that have much larger diameter muzzles than do double rifles. Heavier muzzles throw too much weight forward, toward muzzles, something not needed or desired on a double rifle.