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[rr22]

I was making a 50g batch of these,using the formula;

Bismuth trioxide Dragon eggs Bismuth trioxide or Bismuth subcarbonate 75 Magnalium (-200 mesh) 15 Copper (II) oxide (black) 10 Aluminium (atomized, -200 mesh) +5 Nitrocellulose lacquer 10%

and method from Pyroguide,as I was cutting the rolled out mix on polythene using a scalpel I heard a distintive "crack" I drowned the comp in acetone,and sought advice, (thanks Cooperman).
I think that what happened was some chunks(about 1/4") of Bismuth trioxide got into the mix,and caused a small area of dry comp. that ignited from friction.
Some on passfire speculated that this was just mechanical noise but a sub .01g particle replicates the noise I heard exactly,and it's a pretty unique sound.
In my final analysis I think this boils down to my bad technique(not screening the Bismuth trioxide),but hopefully interesting/instructive.