The issue with BP at high altitude is that when the black powder charge goes off, the black powder goes from being a tightly packed bunch of powder to being a loosely packed, then just loose bunch of individual grains no longer in contact with each other. If that happens at low altitude , the air conducts heat from one grain to another and more of the grains burn. When that happens at higher altitude, the same separation between grains results in lower heat transference and a failure to ignite the entire powder charge.
Jim Jarvis and Tony Alcocer have both done a lot of research and have figured out how to ignite most, if not all of their powder charges, by doing just what you said, containing the charge, not to the point where there’s metal shrapnel, but so the powder is prevented from spreading out before completely combusting. Tony (TFish) really tweaked it by placing the ematch at the top, or open end of the charge container, rather than at the bottom. That allows the charge to burn down into the container rather than blowing the powder out of the container before it can ignite.
