Regarding what use a flattened horn was put to, we see a lot with a spanner for spanning the lock of a wheellock but that does not tell us whether pistol or rifle.
One thing to take into account, is that Germanic arms for hunting were often fairly large calibre, but small charges were used in the fast twist barrels.
This may affect our thinking as to what the flask was designed to do. (By this I mean a small flask will hold more loads than we may think!)
Another thing to take into account, is that most European hunting was a "day job", so no need to pack for a Long Hunt, or carry carry more powder than could possibly be required in a day.
My little horn probably holds for 15 to 20 shots, more than enough for a day's hunt!