It's not whether you have a muzzle loader or cartridge arm. The difference is between smokeless and black powder. When loading cartridges with black powder no air space is allowed, just as with a muzzle loader.
If you could load smokeless in a muzzle loader you might be permitted a small air space just as in a cartridge. Of course, one of the reasons (among many) you don't use smokeless in a muzzle loader is that it would be neccesary to seat the bullet to the exact same position each time or you would have very erratic pressures.