I practiced on a small antelope last year. Just let the head rot a few days and keep twisting around on the horn sheaths. They eventually popped off from the decaying process. Hit them with hot water and then let them dry out. No damage and worked fine. Then let skull do its thing like any other one. I wouldn't hesitate to do a big antelope using that process.
Ackleyfan, that is one seriously big looking antelope, congrats.
