Bull, If you decide to burn it the powder may not flow very well since the hole will be charred and it will have a bad curve to it.
What you will need is a piece of 3/16" metal rod. Be very careful to bend the rod to the exact shape of the horn and remember the direction you had bent it too. It should be to the same curve as the long or outside of the horn. After you have the rod bent to the exact shape of the horn and allow for the curve to bend in because the cavity will end up in the center of the horn not the outside edge. Then sharpen the end to a point and get your propane torch out.
At the tip where you cut it off, drill a 1/4" hole about 1 inch deep, STRAIGHT in toward the cavity center.
Heat the pointed end of the rod, cherry red and quickly stick it into the large end of the horn and force it into the tip and push it into the curve so you will end up burning toward the tip that you drilled.
Oh yeah, one VERY important thing, do this outside
BIG stink and lots of smoke.
You will probably only go about 1/4 inch with each burning, push till it cools and stops and bring it out, reheat and do it again, and again, and again. if you are force it into the curve, eventually, you should end up in the hole you drilled.
This procedure is really open to mistakes and burn outs. I have burnt several with good luck, but it does have the potential of a burn through and you will NEVER get rid of that smell.
Good luck and I hope this helps ya' out. Post pictures of your burning and the end product if ya' can. Good luck.
My writing sometimes does not make sense, so if you need a better explaination on anything just ask.