You're missing a few parts of the equation if you are comparing to a cartridge load. A ML starts with the bucket seated tight in the barrel and against the powder. If you don't do this then you can risk bulging the barrel with even acceptable loads.
With a cartridge load you have space inside of the brass case, as well as the leade for the bullet to jump. This allows for some expansion room and allows for some pressure to build more slowly and avoid the spike. This makes a huge difference, not to mention the trap door loads for 45/70 use a very reduced powder charge which created a lot of expansion room in the casing before the bullet jumps the leade and contacts the lands.
This phenomenon can be seen to some extent by working up a load that has too long of an OAL and getting a big pressure spike.
This is why you can't compare those apples and oranges
The BPCR smokeless loading is only being mentioned to show that 4198 can be used in antique conversion guns made with antique steel.
In the early smokeless era, there were weak smokeless powders and bulk semi-smokeless powders that more approximated black powder characteristics which aren't produced anymore.
One such smokeless was named Schultze powder which can be read about on Chuckhawks. --->>>
https://www.chuckhawks.com/real_blackpowder_substitute.htm
Another was named King's Semi-smokeless.
What is semi-smokeless powder?
"
Semi-
smokeless powder was a mechanical mixture of mostly black
powder and a fairly small proportion of nitrocellulose. It was useful because it could be loaded bulk-for-bulk with black and had similar ballistic properties but left significantly less fouling in the bore." --->>>
https://www.milsurps.com/showthread.php?t=13511#:~:text=Semi-smokeless powder was a,less fouling in the bore
Here's a short interesting thread titled "
Pyroxylin and semi smokeless" alternative powders: --->>>
https://thefiringline.com/forums/showthread.php?t=588661
The point is that since not all smokeless powders and loads are the same, then why try to characterize all of them as having the same amount of danger?
Each smokeless powder charge would need to be judged on its own merits considering the quality of the barrel steel and other individual load factors.
For instance, the Kings semi-smokeless was used in original C&B revolvers that were made using much weaker steel than is used in reproduction C&B revolvers today.
The fact that it was recommended and safely used by experts can't be denied.
The Ruger Old Army was originally proof tested with full chambers of Bullseye powder.
The North American Arms Companion and Super Companion .22 C&B revolvers were originally advertised as safe to be loaded with Bullseye until the Fed's threatened to remove their classification as an antique if they didn't change the recommended load listed in their owner's manuals.
But many people currently load them with 1-2 grains of Bullseye powder depending on which model.
Otherwise the guns have very anemic performance for use as personal protection with BP and substitutes.