Now that I can see this closer I would bet that it is silver soldered on. I have seen several examples of Russian guns with breaches similar to this. I have those photos in my library. Many modern rifles have sight ramps held on in the same manner and it is impossible to tell even under a microscope.
I know Mike Dubber pretty well and he is a superb Engraver and one of the very best in this country. However Every Engraver has his own style that may result in equal results. Among those are Joe Rundell, Ron Smith, Sam Welch, Eric gold, and Alain Lovenberg of Belgium.In my opinion, Joe Rundell is the best sculptor in this country as of now. I don't do too bad. I learned some from most of these guys. In the photo below I silver soldered the shield on the breach before I sculpted the rest of the scrolls and stuff. The temps required to silver solder stuff at the breach end does no harm to the barrel of a black powder gun.
If such high relief applications were not soldered on It would require the removal of a lot of metal for a long distance down the barrel. There is another way. That is the application of a iron inlay or ovewrlay. Many of you would probably say that is impossible but it is not. I learned how from Ron Smith and Frank Hendrix.
There a basically two different methods of sculpting metal. Chiseling and punching. Most use both.
I am going to post some photos of some Russian guns with similar applications.