Coyote Joe: Black powder produces a bell shape pressure curve, beginning slow, and rising quickly but over some time to the peak, and then dropping off slowly as the ball moves down the barrel. Hence the term, " Bell Curve". In contrast, smokeless powder detonates, with the initial explosion sending pressure to the highest point, like the rise of an agena rocket. Then, the pressure drops as the bullet moves forward. The result looks like the shape of a ski jump, instead of a bell.
Light black powder trailed on the ground, and you will see it burn "progressively" as if it were a fuse. Now, closed actions, such as Black Powder cartridges, and many percussion actions attempt to shorten the slow rise of pressure,( and succeed) by using very hot caps or primers, to blast a flame through the center of the powder charge, thereby igniting powder in the barrel or casing from the center out, the length of the chamber from the plug, to the base of the bullet. The only problem with this is that those hot primers are also moving the ball or bullet forward before the powder is able to reach its peak, changing the dimensions of the chamber with each shot, and giving wider SDV, and poorer accuracy down range.
There is a move in the Black Powder Cartridge arena to use large pistol primers to ignite the powder so the primer does not move the bullet too soon. In effect, the shooters are finding that the modern caps and primers are TOO HOT for accurate long range target shooting.
Now, will you notice the difference shooting off-hand at 35 or 50 yds? No, I seriously doubt it. Hunters will generally not have to be concerned about this, and can shoot all the " Magnum" primers and caps their hearts desire. But the BP accuracy boys are looking for more, and are doing a lot of work to get it.
I am recommending to people that they consider using standard caps, or use an over powder wad, and then some kind of filler, or cushion wad, to absord the initial pressure from primer, while the main pressure builds in the chamber behind the PRB or bullet, which then will overcome the inertia of the projectile and send it out of the barrel with better consistency. Let a coarse filler, like corn meal, take any compression caused by a hot cap or primer, so that the bullet does not move until the main pressure pushes the entire load down the barrel. The second benefit of using the wads and fillers is that you protect the base of the bullet from being melted by that hot primer or cap.
Paul
