Jerry, FWIW....sounds to me like you're using a little to much prime...2 to 3 grains of FFFFg is more than enough, an equal amount of FFFg should also do the job.....either way, you shouldn't be hearing, or seeing, that so called "hiss".
On all my flinters, I get a quick flash and a very sudden boooom before I can even think of flinching. After a bit of practice, you will not even notice that flash.
Holding longer, if such can be said, is a great idea and I personally believe it helps although the experts say the ball is already on the way...that may be, but long time rifle shooting has taught me to hold, hold, and mark...meaning you call "exactly" where the shot broke on the target before you even bring the rifle down....
Uncle Russ...
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