Bob,
This is what I do. I mostly use 30.06 shells, but I've done 7.62mm and .34 caliber successfully also.
I drill out the head of the casing to 5/16" on the lathe using drill chucks.
I then cut a length of 5/16" OD brass tubing to match the length of the casing.
I then dimple the brass tubing with a small punch. (I'll explain later)
Then I lightly sand the outside of the tube (probably not necessary), clean with acetone, put a dab of thick CA inside the neck and then insert/press the tube into the casing from the head end. It should be flush on both ends. Any prutrusion can be easily trimmed by hand with countersink bit.
The casing then goes on the lathe to be polished and sealed with white PPP. You'll have to use a 7mm tube as a bushing for inside the casing should you choose to polish on the lathe. (I don't powder coat the casings)
Then I prep the 7mm tube like I would for any other pen; sand and clean. I then press the nib onto the 7mm tube. (I don't use real bullets, just the standard slimline nib)
Lastly, I dab thick CA inside the casing and insert/press the tube into the neck of the casing.
The CA and the dimples combine to hold the lower tube securely in the casing and when the transmission is pressed in, it is very secure.
I then make up the upper barrel, usually antler, and finish with CA.
I thank Monty for suggesting this method as I tried lots of different ways to secure the 7mm tube inside the casing with much frustration and little success. This way works best for me.
My problem is everybody wants to buy my "Deer Hunter" pen (as I call it), but I'm not crazy about making them. I much prefer wood over antler, but I get $50 a pen and my customers don't even flinch. Hmm, maybe I should charge more.
I hope this helps!