All the manufacturers of BP arms, and ALL the loading books, and everybody I've ever spoken to about the subject over the last fifty something years say exactly the same thing, as, indeed, all the demonstrators on YouTube, and everybody I've had contact with on muzzleloadingforum.com, many of whom are professional builders of muzzle loading firearms, often costing into five figures.

When loading ANY BP arm, even using substitutes, you leave an airgap at your own risk, IOW, DON'T DO IT. Short-loading any BP arm is fraught with danger, and that ball or bullet MUST be firmly seated on the charge.

You can argue and ask what if until the cows come home, but if you wish to carry out your own experiments into what happens if you don't, you are on your own. The list of bulged and/or burst barrels and missing body parts stretches from here to there and back again.

I've no more to add to that.