The common inner diameter for 3/8-10 modified is .308 inch. It is best to bore out the hole on a lathe.
Without a proper lathe, you could use a 7.8 mm drill bit, carefully letting the drill bit self center. Then you might still need a tap to clean up the threads.
I have assembled a couple of butts from carbon fiber. I wanted to make a break cue with a 7 ounce shaft and overall weight of 18 ounces. The cue didn’t perform any better than my jump break cue. A friend tried the butt and loves it for regular play.
I made a second one and with a titanium pin it...
Here is another cue I finished recently with burl/fugal damaged maple. It is stabilized and cored. The handle is shedua. The collar and cap are of elk bone.
I feel that CA bonds best with clean flat surfaces. When installing leather tips, like Ultraskins, I sand flat with 220, then apply a layer of CA. I let the glue soak in for a moment, spray it with accelerant then sand flat again before installing.
It all takes time. And when sanding care must...
I install tips. For most tips, I sand them flat before installing.
The Kamui Clear, Rhino Time Crystal and Caiden Fighter have flat glueing surfaces that install well without any sanding.
Are there other brands out there that don’t require sanding to be flat?
Leonard
I have collected some Dufferins to make conversions that I no longer need.
I have both one piece and two piece.
Three are two piece green leafs.
PM your info and I will text or email pictures.