4seventy I have one of these rifles chambered for 9.3X74R, with ejectors, and scope mounts. The individual barrel groups are in the MOA @ 100 yds. The problem is the bottom barrel shoots 5" to the right of point of aim, and the top barrel shoots 5" to the left of point of aim, both at exactly the same elevation. When one looks at the muzzle end, with the rifle held perfectly vertical, you can see the need with the naked eye! The bottom barrel is pointing to the right, and the top barrel is pointing to the left. They need tweeking to place one over the other only.
I re-regulate S/S doubles all the time, but they are soldered with softer solder than silver, in most cases. If the barrel are not off much, I heat the solder from the inside with a fixture I made which utilizes two heated bore size rods inserted in the muzzles about 5", and simply twist to line the barrels up. This usually doesn't even damage the blueing, and works like a charm in most cases, if convergence is OK! I've used this method to re-regulate barrels on old doubles (S/S) where because of wear, one barrel shoots lower than the other, which is the same condition on the O/U, shooting right, and left! Usually takes about 20 minutes to complete!

The heat would be higher with silver, but would this method still work on the barrels that are silver soldered?

I have been tempted to do this on my Grand Europian! It is a very nice rifle, and would make a fine deer, and elk rifle!