If the tang of the shell auger is tapered I would drill the hole about 2
sizes smaller than the largest section of the tapered portion of the tang.
Make sure the ferrule is in place before trying to insert the tang into the
handle if it is not in place when you put the auger tang into the handle it
may split the wood.

I could be wrong about this when it comes to a shell auger but when I have
made file handles that is the way that I make them and it has worked well
for me.