... what Paul said... but before you cut, you can change the size and shape
additionally, remember that you can change the shape of the horn after you get it nice and hot (but not hot enough to cook the horn, which will ruin it). i use cooking oil; check out the book from Track of the Wolf - money well spent, since after you build this horn, you'll just be looking for a reason to do another one.
my next priming horn will be softened and flattened so it will fit better in my hunting pouch. (i know- that's not HC/PC, but i don't really worry too much about that.)
good luck with your project!