There is one other thing to consider.
Accuracy.
Some thinking out loud/theory follows...
The column of powder does not ignite all at once and a certain amount of the powder is pushed up bore behind the projectile. This will form a wad of sorts.
This is indicated by the deep powder grain impressions of the bullet base in conicals or at least in BPCR.
So we have to wonder if this is the case with a PRB if this helps protect the patch during initial acceleration. Since the RB moves far easier than a bullet in a cartridge case or in a ML we have to wonder, especially with heavier loads how much powder is unburned when the ball starts to move.
Yes a looser powder charge will ignite faster but even though the cartridge load should be well settled in the case either by vibration or drop tube and then lightly compressed by a wad the ignition source is far more energetic than flint or traditional percussion cap. Modern primers are pretty hot stuff.
Also if I recall correctly in doing frontal ignition with the 50 BMG during WW-II Elmer Keith wrote of some serious pressure excursions that killed the project. But this was smokeless powder in a cartridge that already operates at high pressure, about 60000 IIRC.
Load inertia. In cartridge guns load inertia is critical it making for a low velocity SD, a low SD under 10 fps is absolutely necessary for any accuracy past about 150 yards. This is done in various ways, seating the bullet hard on the rifling, a high bullet pull and I think the tightly settled powder charge is part of this. Some resort to heavy compression on the powder which while destroying 10% or more of the grain structure will also reduce flame propagation and locks the charge tightly in the case.
So does the charge of powder in a RB rifle increase load inertia? Does the projectile in the front and building pressure at the rear lock the powder grains against the bore for a millisecond increasing the load inertia and improving the burn and/or ballistic uniformity? Would front ignition change this? Would it make things better or worse?
Initial acceleration of the bullet by BP is far more energetic than even a relatively fast smokeless powder like Unique. It will accelerate a bullet fast enough that even one .010-.012" under groove diameter will not gas cut. It is upset to seal the bore before the gases reach the bullet base.
Yes, cartridge guns and round balls are apples and oranges or very close to it and things that are important to a rifle shooting 500-1000 yards are far less important to one shooting to 100. But all these things need to be considered.
I suspect that the amount of experimentation done by the British in the 18th and 19th century that this would have been looked at but???
Its a fun experiment but I would use a 4150 barrel for this.
Dan
