Trapdoor Rifles

In the article John S posted there are two sectional views of gallery loads. They both have I believe 5 gr. BP with no filler. I asked John about the filler and was told that the sectional views are as the round was made.
I’ve been taught and was hard pressed on the fact that there should be no air space in a black powder round or in a muzzle loader so I’m kind of puzzled at the fact that no filler was used???
Anyone here have any thoughts on this?
Could it be that the charge is so small that there is no catastrophic result

Do not confuse U.S. Army Ordnance Department loading specifications with the modern day recommendations of reloaders who have the advantage of multiple instruments and tests not available to the OD back then.
Also, remember, Gallery Practice loads were intended for use almost exclusively on indoor ranges at 50 feet, and intended to be reloaded at the unit level by PVT Beetle Bailey's grandfather.

Yes, filler is the modern best practice.