Awhile back I built a Hawken which had a 1" X 7/8"x 36" bbl in .54 cal. for a customer and last fall he shot a nice bull elk w/ it. It's a very accurate rifle besides being a joy to carry on a full day's hunt in the Rockies.
Seeing nearly all BP bbls are quite hefty compared to CF bbls, I don't think that harmonics play much of a part in achieving excellent accuracy w/ BP guns. ...also, pressures are much lower w/ BP compared to CFs which results in a lack of harmonics or very much reduced harmonics.
When shooting my first BP rifle which was a TC Hawken, I was surprised how easy it was to sight in this rifle and it stayed that way throughout its use. Started out using it for squirrels and seeing I wanted 6 pcs of meat from each squirrel, head hits were mandatory. This rifle later on also shot deer and elk.
Harmonics in BP guns can really be ignored because of the hefty bbls and the lower pressure loads......Fred