the only accurate comparison between a modern projectile and a round ball would be if you had the same weight and same velocity
Well the weights are quite close between the 170 grain .30-30 and the 177-180 grain .490 round ball, as well as the heavier bullets available for the .35 Remington and the 225 grain .530 round ball. The .357 magnum "works" with a light bullet for the .390 RB and with medium weight bullets for the .440 RB. The specific cartridges are mentioned as they are often "familiar" to the modern shooter who is part of the conversation. All the cartridges are factory loaded faster than the BP loads will perform in most barrels, but the reloading data shows the bullet weights and velocities do cross when hand loading. Many of the .357 factory loads from rifles with 20" barrels actually are under velocity when compared to the .390 or .440 BP loads. (Usually the person who is not well versed with BP is asked if they would consider hunting deer at 100 yards or less with a .30-30, a .35 Remington, or even a .357 Magnum in a lever-gun..., and the conversation continues from there.)
AND as was pointed out..., it is a
crude comparison, and done for those not well versed in BP performance. For, as is well known, even when using all lead bullets, the cartridge bullets are harder and have thinner cross sections so penetrate better but mushroom or deform less. Still there have been those who on other forums or other threads have wondered about the .530 RB as it's very close to the projectile weight and diameter of a .45 ACP. I use it as a way of getting the ill informed to understand that there is much more to it than caliber.
And shot placement is the key to both black powder as well as cartridge guns.
LD