A combination gun such as you describe was designed to be used by a jaeger as a rifle rather than a game gun. The shotgun barrel was there to deal with a fox, marder, or even stray cat while the hunter sat a high seat for roe or boar. Fit, in the British sense of the word, wasn't particularly relevent.
It was (is) more important on a drive hunt where feather and fur would be encountered. For such a hunt, the drilling was the better solution. In that case, fit would be more important for use of the shotgun. But even in this case, the rifle would not have been an issue. For that boar at 70 meters, one needs to "aim" that rifle, not point it.