This has been pursued by others and the problems are more than just pressure levels. Smokeless burns a lot differently than the muzzleloading powders and the pressure curves are way different. The additional problems with erosion of the nipple and breechplug due to the temperatures at initial light off of smokeless are other things to consider. There are those that claim they have been doing just what you considered but it is definately a very questionable experiment. Personally I do a lot of reloading and shoot some smokeless in my 40-65 but it is modern steel and there is a cartridge holding the powder. Compression of smokeless is another thing that you can or can not have problems with so loading a bullet on top of a smokeless charge not knowing how that would effect the burn rate and pressure rise again causes severe questions as to the safety aspect of such experimentation. Notice that twice I mentioned this as an experiment and I am not into experimenting with things that explode CAUSING SEVERE INJURY IN THE ACT.