Smokeless powders develop much more pressure than BP and BP substitutes....that and they are MUCH more fussy about the accuracy of your measurement. A few grains too much and the pressure goes up significantly!
To be brief, they physics behind an explosion encased in a cartridge versus right against a plain ML barrel with a touchhole is very different as well.
Finally, smokeless powders have a LOT of variation. You could really get yourself in trouble trying the wrong one.
There is one ML designed for smokeless on the mass market, and even one of those blew this year.