Truing The Top Cylinder Deck
The idea is to be able to hold the cylinder tightly so it can be trued - that's it. If you use this tool to hold your cylinder, you may want to dress the bottom surface of the cylinder. But, after you do that, you must then rebore the cylinder. The bore has to be true (perpendicular) both fore and aft, and right to left to the base surface. This is the only way to do this.
If you're only going to machine the top of the barrel, you can hold it in the fixture without redoing the bore.
Actually, it's pretty easy to just grab the barrel by the bore. You can remove the head studs and put it in the lathe just tight enough to hold it while you tap it into being as true as possible. Then, tighten it and clean up the surface making 1 or 2 very light cuts. It usually never takes more than .001" or .002" total to clean up even a really badly scored surface. Making a .003" cut would be unusual.
If you want to get into doing things like stainless o-ringing the top of the barrel, you will need to hold the barrel the other way - from the cylinder base. The tool you are thinking of making may be ok, but I always found it necessary to be able to move a barrel a little, to be able to put the groove in the right place. But, you'll have to see how it goes.
With the base trued, you can bore the barrel. With a good bore, you can add the ring groove.
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