I'm pleased to find this forum and as a newbie here I'll simply say that in the past I did a whale of a lot of posting at several ML forums.
So allow me to politely jump in with both feet here. Contrary to common knowledge, having space between powder and bullet is not an "obstruction" situation, and instead creates, via internal-ballistics principles, a larger powder-volume space. That means lower, not higher pressure is created by the ignition of the powder. There is, very rarely, an issue of "detonation", a poorly understood phenomenon in smokeless powder rifle loads wherein the primer flash might travel over the surface of a "leveled-out" powder load and thus ignite it far too fast, creating a highly excessive pressure peak. But note that is smokeless powder, and very rare.
So, using BP in caplock or flintlock MLs, or in an inline rifle, or Pyrodex, Triple7 or Blackhorn 209, will mean poor pressure buildup if the bullet migrates forward of the powder charge. And with the marvelous pressure capabilties of BH 209, its tendency is to ignite poorly if there is diminished restriction to its burning, such as a loose bullet, oversized bore, too-small sabot, etc. That means lower, not higher, pressure. Additionally, for what it's worth, BH 209 has a virtual duplicate of BP's pressure curve, meaning it is a very quick peak followed by rapid pressure drop-off. It also means that BH 209 is more likely to produce bullet oburation and hence fill the rifling grooves, than the other BP substitutes do, and gives it the remarkably low ES (extreme spread of velocity) in any given string of shots (provided that the bullet is tight in the bore via its initial diameter, sabot, skirt diameter, or bullet obturation at ignition).
In the later 1840s, there were some european MLs with typical soft iron barrels that had an iron post projecting forward from the breechplug (Delvigny style), intentionally creating a space between powder charge and bullet!
As far as undersized bullets, the ML ones that look jacketed are instead soft lead that has a thin copper coating to lessen fouling and look nice (read, cost an excessive amount). They are designed to obturate upon ignition of the powder (yes, that happens before the bullet even starts to move, which was proven by Dr. Franklin Mann in his 1902 book).
In closing, blown up barrels are virtually always caused by an actual obstruction in the barrel, which a fired bullet or shot charge meets, thus producing an instant pressure spike that may blow up the barrel.
Aloha, Ka'imiloa
what distance ?
