AS pointed out, a priming horn isn't much good in a cap gun, except maybe to trickle some FFFFg into the nipple it it doesn't go bang. But it sure is a cutey, maybe enough to carry in a pocket when out to slay the wily deer....
No Powder, I tried various priming vessels over time, and always had problems with that fine powder clumping in them. I now prime from the horn, but if I DID need a separate container for priming powder, I would get out my old nose medicine squeeze bottle with the guts removed. Maybe not PC, but I never had any problems with moisture getting in.
~Kees~