I'm still not sure what you are attempting but since we are on the subject of deep holes it is common practice to convert the process in things like lamps by cutting the part in half and then cutting the grooves in each half. You end up with a square rather then a circle so depending on the use this might or might not work for you.
The advantage of doing it this way is that the resulting hole will be centered and straight where as with a drill situation it is unlikely that it will be even close to centered or straight at this length. Wood is likely to have grain patterns and possible hard/soft places as well as knots and other non-uniform transgressions that the bits will not deal with very well.
Here is a quick look at how this works:
ScreenHunter 56.jpg (48.79 KiB) Viewed 2811 times
No idea if this will work for you but I believe if it will then it is the best way to go rather then the drilling.
Ed
{Knight of the Shopsmith} [Hero's don't wear capes, they wear dog tags]
