Great question and concern, but 'done right', you're well away from the crown. Yes a dimple is a displacement of metal, but that's only in the immediate area where the metal is raised/deformed.
I'm not at my home PC, but I have an extensive 'collection' of muzzle decorations that go far beyond dimples. Some examples have '0's put around the perimeter on octagonal barrels, where the flat is the widest (again, to protect against the concern raised above). Others combine the dimple with a scribed circle on the outside of the dimples.
I once made a DIY dimpling jig to do that line, plus even the dimples, so they look at the same height. I took a 6" dowel that had a flat filed onto it and I securely taped a 'prick' punch (a sharp 30 to 60-degree included angle tool) to it. Prior to peening, or dimpling, I did my best to mark with a fine point Sharpie, where I wanted the dimples to be. Using magnifying visors, I adjusted the 'flat' of the dowel so that the dimples appeared to be centered between the bore ID and the muzzle, about mid-point is fine. The dowel here helps you maintain that correct position, having some in the bore to support the punch and some above to help act like a handle. Make sure the striking end of the punch is above the dowel though ... LOL! Once peened with the 'prick' punch, I followed up manually (but wearing visors) with a 'center' punch (a wider 90-degree included angle tool) to make them bigger.
To scribe the circular line, I modified an older prick punch by carefully grinding an angled flat radially to the tip so that it would cut like a single point cutting tool when rotated into the work/muzzle. My dowel was a little more elaborate here this time, where the dowel started out a full bore size, then I slowly chiseled a groove (not a flat) to hold the punch securely (all sides securely against the dowel, if you can, i.e., bore size dependent, then positioned at the height I wanted the line to be off the bore. Go SLOWLY here ... but it works!
On coned barrels, I also LOVE the ones where the grooves are filed into a decorative almost 'flower' looking design at the muzzle end. And yes ... such barrels STILL shoot as well as they did before! I'll see if I can add a few of the common designs later ...
