I've drilled on a horn that was filled with wood a couple times, worked out fine. The drill bit broke up the wood pretty easy. COOOLL pic Del! That's something you could use in a book or something. Very cool! I have gotten pretty crazy with my drill bit nowadays. I sharpen the edges like a knife, I go from wet stone, to leather strop with a bit of polish, back to the wet stone, then back to the strop, etc. After I am done, I take my carbide thingy I use to roll the edges of my scraper, and roll the sharpened edges just like a scraper. After that, it tears through the horn like crazy.
It makes lots of shavings instead of the dust it used to before I used to roll the edges.