I've got an octagon cutting jig for my mill, but haven't run a machine in over 5 years. The jig is ingenous, I didn't make it. It consists of a heavy flat plate with a bunch of holes tapped into it so that you can attach a tailstock and an indexing head at different distances. In between the two centers, there is a series of holes tapped in a straight line. One center is adjustable for height, so that the taper can be adjusted. The other is a collet held in an indexing head. You start by machining a round tenon at the breech end to go into the collet. Mount the collet and tailstock at the length of the barrel, put the breech tenon in the collet, and put the tailstock center in the muzzle. Snug the barrel in and adjust the center height for the taper you want. Now, there are a series of spring loaded pistons that screw into the intermediate holes between the centers. You let the springs firm the piston up against the barrel underneath it, and then lock them in place. This supports the barrel rigidly every couple of inches. In practice, I use a fly cutter and take fairly small cuts. A large end mill would work as well. I cut a few passes, then index the barrel over 180 degrees, and reset the spring piston supports. Make an identical cut to the first. Now, rotate 90 degrees, cut, rotate 180 degrees, then 45, then 180, etc. When you have cut all 8 flats the first cut, repeat the process for a few more passes on each flat again. Repeat until you have it cut to form. The beauty of this jig is the support. It keeps the barrel from being bowed under the cutter force. I wish I had someone I could permanently lend the jig to near me, with the condition that I could go use it on their mill. It weighs about 140 pounds, and is a pain to set up on my small machine, and I don't have enough travel to use it effectively. Luckily, I can slide it on my table to cut most barrels in two passes. Draw filing cleans up the overlap. dave
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