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Cap'n, when I first looked down the barrel, I thought that it was unburnt powder. I placed a plastic bag over the muzzle and ports and used a light charge of compressed air to collect a sample. I've seen a lot of unburnt powder in my life, and this is not powder. When you touch a flake with the tip of a dampened toothpick, it dissolves. Any unburnt powder will not do that.
Now that's interesting. The residue must be powder ashes then, but I still think your load would benefit from an over-powder seal. Don't be fooled by the pressure needed to seat the wads; the lips of a plastic seal will still expand and seal much better than a felt wad (and a nitro card doesn't seal at all until it reach the bore). Winchester/Olin found that a bottle cap shaped paper card provided a much better seal than normal wads as the pressure would help expand the lips of the seal against the walls of the shell in the chamber (which is a critical point as the pressure spikes long before the payload has entered the bore).