Years ago, some of my buddies started hunting doing a lot of deer in a state that required shotguns with slugs, which I have done a lot of since then. This was well before the Hastings rifled shotgun barrel became available, and they hadn't yet started loading sabot slugs. What these guys did was to wrap the entire length of the shotgun barrel that stuck out beyond the fore-end with black electrician's tape. The whole arrangement ended up being about a big around as a toilet paper tube, and they said that it worked great to dampen the vibration of the barrel. They were getting 2-inch groups at 50 yards, with smooth-bore shotguns with choke.
I haven't tried this with a rifle, but I have to think that the barrel-dampening concept works. This shotgun trick is admittedly an extreme example of applying the concept. Like everything else in shooting, there are lots of variables in the equation, and barrel vibrations & harmonics are just one of them.