You won't be able to take the compound or twist out or the horn! Do you know if it's a American Bison (Buffalo) or Asian? Generally, American bison don't have a twist. It doesn't really matter, as it sounds like a good canidate for some fancy spout carving!
run a stiff wire up into the horn and then pull it out and hold against the horn and mark where the cavity ends. Now measure from that mark about 21/2 to 3 inches and cut off tip. Use a marking pen and mark a dot in the center of the horn tip where the hole is going to be. At the tip, mark four lines about three to four inches long on four sides of your horn spout end. Use these lines to "eyeball" your drill bit to see that it is running straight as you drill. Go slow with a 1/8" bit in and out till your feel it coming out of the cavity, then enlarge your drill bits, and ream out until you get the hole size you want! You only have to heat the big end for rounding, not the tip end.