superspirit, it's a shame you didn't get good accuracy with the 12.6 load with the CCI-500 and flash should have been less than with the same charge with a CCI-550. Your accuracy improvement may have just come from settling in with the recoil because of the low number of rounds you made and tested. Bullseye is a flake powder and at high pressure, flake powders flash. The higher the pressure the greater the flash will be.
I am working up similar loads but for a Ruger Speed-Six that has a 2 3/4" barrel. My powder preference would be AA#7 but I don't have any. I do have Ramshot True Blue and their ballistician recommended its use for this application because its not as sensitive to powder positioning. With lighter charges of fast burning powder, chargeweights will be low so powder position can be an issue with .357 Magnum loads. TiteGroup gets recommended for these types of loads, but I do not like TiteGroup personally because it's known to spike at higher pressure. True Blue is slower and my loads are at about 50% density. But because True Blue is a very stable powder that gives very uniform ballistics in all handgun cartridges, I'm gonna put it to the test over the chrono. If accuracy is as good as I think it will be and the ballistic stats show that it is uniform at around Lyman's start charge data, I plan to use it in place of #7. True Blue is a very dense spherical powder that meters like hourglass sand and flash is very low in high pressure cartidges. May be just what the doctor ordered. Try it if you can get it, or AA#7 that's worked extremely well for me in the past. V-V 3N37 is another good one, but it's a bit expensive.
