When loading black powder guns, you must always seat the projectile hard against the powder charge, no matter what. Never, ever, ever leave an air space between powder and bullet, or it could create a pressure spike and blow your gun to smithereens.
When loading smokeless powder, never, ever seat the bullet too deep, even if there’s a huge air space in there (38 Spl comes to mind) or it could create a pressure spike and blow your gun to smithereens.
You should never, ever use smokeless powder in a black powder gun, because it could create a pressure spike and blow your gun to smithereens.
If you’re loading smokeless powder in a metal cartridge case designed for black powder, to be loaded into a gun designed for black powder cartridges, it is not only OK, it is recommended, and universally used both by hand loaders and ammunition manufacturers. Using black powder in a black powder metal cartridge is a relatively rare, esoteric art. So rare in fact that the loading manuals almost never mention doing it. It will dirty up your gun, so always use smokeless unless you just want to make some smoke and be a show-off.
Smokeless will blow my percussion revolver to smithereens! Unless I install a cartridge conversion cylinder, in which case it will be fine with thousands of 45 Colt smokeless loads.
So can I take from all that, assuming it’s all true, that I can safely use smokeless powder in my 1858 Remington percussion revolver, using the percussion cylinder, so long as I observe loading data for, say, the 44 Russian cartridge, and be SURE to leave a sizeable air gap between powder and ball? Or is something in the above paragraphs not true? Surely it’s either/or.
Not that I intend to try it, or that I even want to try it, mind you, but to make a point about Common Wisdom.