Guys,
Damn I hate to be the class know-it-all (I don't) but in real life I'm a physicist and working out problems with interacting systems, in the immortal words of Father Guido Sarduci, "is'a my beat." I've long worried that other shooters don't realize how delicate and close to disaster the black powder firearm is every time we loose a round.
Ballistics is not my field but I have access to enough data that I am able to "run the numbers" tomorrow and publish the equations for any interested. The problem stems from the chance of the so called priming powder mixing with the main charge and therefore grossly altering the burn rate for the entire charge.
Worse is the following: The safety of the black powder system relies on a known burn rate for the powder used and corresponding pressure peak and curve for that powder. The problem scenario goes like this: I fire the lock and the 4gr of #ffffg that I've used for "priming" under the main charge goes off with a pop, burning much faster than the #ffg main charge. That pop produces a corresponding volume of gas which, if large enough, could displace the main charge and the ball/shot load up the barrel a bit. Now, if that bit is large enough to allow the main charge a place to shove some gas pressure as the burn propagates through the stack thereby creating a pressure shock wave that rises faster than the projectile load can accelerate down the barrel and POP it achieves a pressure greater than the barrel's elastic failure point and the barrel fails. Depending on where the projectile stopped that failure could be close to the shooter's left hand, you know, the one that used to be on top of his golf grip.
My guess is that this rather morbid situation is approached every time somebody "primes" their main charge but the planets just didn't line up, at least not that time. Maybe next time.
Like I said I'm not a ballistics engineer so what the hell do I know? Well I know this: I ain't gonna use anything down my smoke hole but the largest grind black or equivalent powder that will burn there and save the ffffg for my pan.
Other engineer or physicists, rip me up if you disagree please. Since lots of beginners read these pages it is morally and ethically incumbent on each of us to make certain that the stuff we publish here is accurate. Rule: If you don't know something for sure, say so. Like I tell helpers, "Tell me what you know or tell me what you think but make damned sure you know the difference."
We owe it to each other and newbie readers to get it right or don't publish.
I warned you that I could get preachy if not restrained,
Rob