All of my CPA black powder rifles, have on occasions, shot five shot groups under 1" at 100 meters. The rifles were the standard silhouette model, the calibers were 40-60, 45-70, 45-90, 38-56 and 40-60 Maynard. All groups were shot using a front rest, no sled, and a rear sandbag, also they were shot using a Lyman STS in 25 X.
Going back over all these targets the common denominator was the weather, all were shot on good days.
The very best group was .560" for five shots.
As for the best powder I can detect no difference on the target between Swiss #3 ( 2F ) and Wano PP. There is a difference however in the strength, 70 gr of Swiss gave an average MV of 1280.7 fps, 77 gr of PP in the 45-90, gave an average of 1169.4 on the same day. Bullet was 520 gr PJ Creedmore with a Federal L/M Pistol primer, no neck tension.
Perhaps there is something in the Swiss powder other than the original three original ingredients perhaps. ? just speculating.
Mike.

I would suggest shooting your BP 250s at Etna Green or an affiliated club. Surely ASSRA would add a note indicating BP was used. ASSRA allows any safe load and they keep records. 

         Joe.