i have always built cartridges and loaded muzzleloaders with black powder and no air space 'tween powder and projectile.
i have never experienced ringed or blown barrels and have only seen images in articles that show ringed and blown barrels that their authors have attributed to black powder air space.
myth? i have no hard evidence, but i'm not dumb enuf to take any chances in a possible myth being a myth until someone shows proof of it all being said myth.