Why substitute powder might not shoot conicals well

[William]

I was gathering information on way to polish the bore on a new Lyman barrel when I re-read this article, which explains why subs may not shoot mini type conicals well;  The reader will notice that BLACK POWDER has been specified for firelapping muzzleloaders. The reason is this: the black powder substitutes, while generating the same basic pressures as that of original black powder, do so with a pressure curve that more closely resembles modern smokeless powders, where the pressure rises in an exaggerated bell-type curve, and doesn't reach peak pressure until the projectile is well on its way down the bore. Consequently, the black powder substitutes do not expand the skirt of that lapping bullet until it is partly traversed the barrel, thus no effective lapping action takes place in the barrel for that portion up until the skirt expands when full pressure finally hits the bullet. Contrast that with the quick "swat" that real black powder develops, where pressure is almost instantly spiked upon ignition, and the skirt of the hollow based lapping bullet is immediately expanded, fully engaging the barrel and lapping the bore uniformly throughout its entire length.

http://www.beartoothbullets.com/tech_notes/archive_tech_notes.htm/48

[mongrel]

This is very interesting, and something I didn't know. One would have assumed that since these propellants were intended as substitutes for black powder, for use in black powder firearms, their explosive/burning characteristics would duplicate those of BP. Apparently not so, which goes to show why it's unwise and potentially dangerous to assume anything when it comes to guns -- of any type.

It's always been understood that one's projectile, be it conical, sabot, or PRB, needs to be firmly-seated against the powder charge not only at the time the gun is loaded but at the time it's actually fired (I specify "at the time it's actually fired" due to knowing of certain types of projectiles, particularly sabots, having slipped forward in the bore during the carry and handling that's normal while hunting). This would be of special concern if using any of the substitute powders to which William's quoted info refers. Black powder explodes, with its pressure immediately beginning to diminish; smokeless, and apparently these substitute BP products, actually burns, and as the quoted information states the pressures rise throughout the process. Having a ball or bullet some distance up the barrel would seem to be a bad situation made even more dangerous by the fact that, while the pressure of a black powder charge would be somewhat diminished at the time it hit the projectile, the pressure of a charge of substitute powder would be increasing and possibly at full strength when meeting the base of the ball or bullet.

This is not to suggest that a rifle loaded with a substitute powder becomes a potential bomb if one is careless in loading -- actually it does, but so does a rifle loaded with real black powder. It just seems logical that there would be a point at which a charge of a substitute would be more likely to ruin a barrel and possibly the shooter and/or any bystanders, just due to the difference in pressure characteristics.