Have you experimented with the distance you have your screens set at? With a snub nose gun, that could make a difference.

Like Power Pistol, I've found HS-6 to be a great powder, capable of good accuracy and velocity, but within a pretty narrow spectrum in comparison with other powders as for versatility. Magnum primers are usually recommended with HS-6. I've compared standard and magnum and could see little if any difference in velocity or accuracy.

This is based on personal experience only; no links.