If I were doing it, I would have the rod blank run all of the way to the butt of the rod. That way, the rear grip is lighter due to the hole running through it and there are no discontinuity with respect to the blank providing feel and strength all of the way to the butt of the rod.
As Phil said, if the hole is bored on the lathe, before the grip is turned, it is trivial to maintain the integrity of the grip with respect to a uniform hole through the full length of the grip.
But, in any event, if the drill bit is held stationary and the grip is turned by the lathe headstock and chuck, it is pretty easy to bore a straight hole through a long length.
I turned these rod grips some years ago. I also bored the center holes after completing the turning of the grip.
The headstock chuck kept one end of the grip stationary. The rock solid rod rest - kept the other end of the grip stationaryl
Then, when the drill bit was advanced into the spinning grip - the hole had no choice except to come out perfectly straight.
[
www.rodbuilding.org]
The key part of the equation is to keep the drill bit stationary, and spin the work piece. That, by its very nature will insure a perfectly centered hole in the piece of work.
Take care
