Thank you Lou for your comments regarding sandpaper on the pen mill.
Some of my comments on squaring barrels:
Most disk sanders are not precision tools. The table is held to a bolt or rod that sticks out from the motor or base, so it tends to flex. For this reason, even a little pressure on the table when sanding and you are no longer sanding square.
Unless you can drill a perfectly centered hole, and the blank is absolutely square, the miter that comes with a disk sander is useless. I set mine up the best I can, and then keep an eye while sanding, so I can compensate by hand.
The miter slot jig mentioned above works well IF the table has been accurately set.
Best CHEAP sander I found is from HF, 5" disk-1" belt sander. I bought it on sale for $40. The table is more solidly connected to the base ON BOTH ENDS.
My theory, and this is my opinion of course, is that it is much easier to line up the cutting surface if it is connected to the rod that goes into the tube. This is how pen mills work. Very simple concept. Very accurate too. BUT the edge does not stay sharp very long.
Two more problems with pen mills:
First, they were designed to TRIM the ends of a barrel that is cut and drilled very accurately. Such as a 2-1/32" tube in a 2-1/16" barrel. Thus very little wood is cut. No need for a power tool (drill or drill press) to use it. The more wood you have to cut the sooner it gets dull.
Second, I don't like using a mill in a power tool. I rather put a wood handle on it. THIS IS MY PERSONAL OPINION. I respect yours if it is diffeent, OK ?
I use my disk sander to get close to the tube. I use the miter as a guide, but keep an eye on the brass tube. I compensate by eye and hand. The last little bit of sanding is done with my "sandpaper on a shaft" jig. This is the same as reversing the pen mill, and using PSA sandpaper on the flat side. If you are going to do this, by all means put a wood handle on the shaft, it makes a world of difference.
In practice it takes me a lot less time to square the barrel ends this way. A little on the sander, and a couple turns with the jig. Also saves me a lot of time messing with a dull pen mill.
NOTE: I just explained the way I square barrels. By no means am I implying that other ways are incorrect, OK ? I took the time to explain this in case you have problems and want to try a new way. If the way you do it works for you, then it is the correct way for you. Enough said! My best regards to everyone !!!
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