The reason one never shoots smokeless in BP guns is 2 fold.
1. Burn rate...
lay black powder on a paper plate and toss in a match.. and you get a Woooff! and a cloud of smoke. Do the same with smokeless powder and you get a Fizzzelllll. Why is this? Because black powder is a very low order explosive. An explosive that has a very slow burn rate and will only pressure up to a certain point.. and it will maintain that until it burns out. Which is why they make fireworks out of it. If you were to fill the barrel of a modern muzzle loader and hammer in two bullets... then light a fuse in the flash hole... it will spin around and around at a high speed until the powder is consumed. It will not develop enough pressure to rupture the modern barrel.
Smokeless powder on the other hand, is a propellent. A inert compound that has been selected to soak nitroglycerine into. Then its covered with a coating that lets the nitro explode.. at a very diminished rate, depending on the type of powder. The big thing is,,. the more pressure that it develops.. the faster it burns. The pressure spike that it develops can be huge!! The size if the grains, the shape of the grains, the composition of the grains and the coating are how different speeds and pressures are concocted to meet certain criteria. Bullseye pistol powder versus 4831 for example. Bullseye is designed to react very quickly, but there is surprisingly little nitro in it. 4831 on the other hand, has a lot of nitro in it, but its designed by shape/size and coating to build up a LOT of pressure, but at a much slower rate as it has far more nitro in it.
2. Pressure spike.
Black powder will only develop a small amount of pressure, no matter how tightly it is contained. It burns at a pretty slow speed all things considered. When its set on the paper plate.. it goes Wooff!!.. but it will never go beyond a certain pressure and it takes time to burn.
Smokeless powder on the other hand, when burned in a closed container.. the reaction will generate huge pressures in a very short amount of time. The more pressure and the tougher the closed container.. the more pressure it will generate
Black powder burns 560 feet per second for very coarse granulations to 2,070 feet per second for the finer granulations.. Maximum. So the pressure wave is pretty slow. and it will never get any faster and it will not create enough hot case at high pressures. Pressure is self limiting.
Smokeless powder on the other hand.. due to its grain size and shape and how much nitro it has in it ...burns faster and faster as the grains get smaller and smaller
Smokeless powders burn on the order of 13kfps to 21kfps. Thats 13,000 feet per second to 21,000 feet per second and as it burns and the grains get smaller..it burns faster. Hence we have a very HIGH and FAST pressure wave. It can generate 150,000 pounds per square inch doing this.
Old black powder barrels were often Damascus. A long strip of iron was heated red hot and wrapped around a rod and hammered to weld the twist together..more or less. The pressure spike and the speed it hit were low. Which is why a true Damascus shotgun barrel can burst even with a low power AA shotgun shell fired in it. The weld cannot adjust fast enough as that high pressure builds up in a very short amount of time.
Modern muzzle loader barrels are often a low grade of steel and while they may not burst with SMALL smokeless charges fired in it.. that FAST and HIGH pressure wave can fatique the steel, along with any attachment points tied to it and ultimately.. it can split or blow off stocks and hardware with bigger charges.
Yes, in the "good old days" large cartridges such as 8 ga rifles (think Africa) often had a cordite (smokeless) base charge.. a small one.. tiny one.. that was used to ignite the big 1 or 2 oz black powder charge ahead of it. But cordite was a low order explosive slightly faster than BP and it could generate more pressure quicker. "Duplex" loads were a big thing several times in history. And some horrific deaths and injuries came about while using it. So powder manufactures have pretty much stopped using it. A duplex load is again, a faster powder close to the primer and used to set off the main propellent charge. The big problem is... over time, simple vibration will cause those two powders to mix.. and then all sorts of unexpected things can happen. One of which is the bullet making it about 20 feet out of the end of the barrel and falling to the ground in a dust cloud of unburned powder. The other is to over pressure the main charge and blowing out the chamber and launching the barrel down range. Really bad juju.
I would expect any modern muzzle loading barrel to be able to withstand something on the order of a 38 special pressure curve, and not blow up. Maybe even higher. But now we have to worry bout the nipple blowing off, the bolster being launched out the side of the rifle and so on and so forth. All that pressure is being held in by simple small screws and not contained in a tightly sealed cartridge well supported by the cylinder or barrel. And because the high heat that is generated by modern propellents.. we would obviously have flame cutting when directed out a flash hole etc etc.
Best one simply shoots what the gun was designed to shoot. Any smokeless firearm can shoot black powder with no issue except a crap load of smoke and soot. The opposite is NOT true.
At this point.. I suppose I should mention the powder used to make blanks in modern cartridges. That stuff has a goodly quantity of nitro and no retardants. That stuff is on the bleeding edge of high explosives. It burns FAST!!! and with a LOT! of pressure.
I attended the funeral of the son of an aquaintence who had loaded 20 ga shotgun shells properly...but.. his dad had locked up all the gun powder so the now very dead kid had taken 308 military blank ammo, popped the red cap out of the blanks and filled the powder measure with that blank powder. Hey... its gun powder... right? His first shot out of his little Stevens single barreled 20 ga blew up the gun, and blew most of the front of his skull off, along with the front right quarter of his brain. DRT. Dead. Right. There. The forensic surgeon (a fellow shooter) estimated that the primer popped and immediately developed about 200,000 psi in .003 seconds. It was a closed casket funeral.
Addenum:
A single-base powder contains nitrocellulose, whereas a double-base powder contains nitrocellulose and nitroglycerine. Nitroguanidine can also be included as a third energetic base, forming a triple-base propellant
In some cases, nitroglycerine can be replaced by nitroglycol because it is more stable at lower temperatures than nitroglycerine
(I used Nitro in the above paragraphs to simplify things.)
