I think the unburned powder "myth"(?) comes from shooting FG in slug guns (see "The ML Caplock Rifle" by Roberts). I have wondered about this over the years.
It's another flip-flop thing for me. I can't see this working but still the man who told Roberts of this was no fool.
I tried it as a kid with a 32 with PRB and fffg. No powder (maybe some fouling but its been quite some time since I tried this) on white sheets at 65 grains where I quite. But then we have to ask how far will the powder grains travel?
The cloud of gas from the muzzle is very hot and I would think it would envelope the powder and ignite it if made it out the barrel unburned.
The last thing to consider is that
Brockway was pulling Roberts leg.
These guys were very competitive and sometimes major $ and reputation was involved in a match so?
I find it unlikely that shooting all the powder a barrel will burn will produce the most accurate load.
Then we have the fact that fast draw artists and some cowboy action types use handloaded BP blanks to burst balloons.... Use coarse powder like FG.... But this is in a pistol.
Questions, questions.
After thinking about this again this A.M. I think the "pulling Roberts leg" fits as well as anything.
But one of you inquiring mind types can feel free to test this anytime you like
Dan