jdavis,
On the subject of setting up a barrel for work in the lathe.....I started out using a 4 jaw chuck, putting a tightly turned brass plug in the bore, and spending time with a dial indicator getting the barrel on center. Before doing all of this, on the far end of the head stock, I had made up a bunch of PVC collars that slid into the head stock hole and had different bore diameters. I would pick one the right size to keep the far end of the barrel very close to center. One day, I didn't have the time to monkey with the four jaw routine and the barrel I needed to breech was a straight 13/16". I had my 5C collet arrangement set up in the lathe, so I just used a 13/16 square collet to hold the barrel on center at the working end and the PVC collar on the outside end of the head stock. This worked great and it was quick to set up. Eventually, I was having some precision wire EDM work done for my normal rocket propulsion test work and I just had the wire EDM house cut a bunch of square collets into octagonal collets. These give a better grip on the barrel and are a little more precise. Here are some pictures:
PS Hungry Horse......I get exactly what you are saying. While I agree with Dan and Scota that one should do the best job possible, one can worry too much about perfection where it is not really needed. I think JerryWH, and others, have shown pictures of a plethora of original breech plugs that would cause some here to regurgitate on their keyboards. Nonetheless those plugs had long service lives, didn't rust into oblivion, and didn't blow out to blind or kill the operator. Now I'm not saying that a sloppy fit is a good idea, but on the other hand, I have read innumerable posts about how imperative it is to "seal" the breech against 10,000 to 20,000 psi hot gasses by tightly butting a flat plug face against a steel shoulder. In my rocket engine designer and fabricator life I have to ABSOLUTELY seal very hot gasses (i.e.~6,000 degrees F) and at pressures on the same order of magnitude or the engine will destroy itself in milliseconds. As a consequence, I am familiar with a myriad of ways to get that type of hot gas seal. I can tell you unequivocally that a flat plug against a stepped shoulder cannot do that job. Black powder breech plugs
appear to seal well for three reasons: 1) The pressure they see is not sustained but builds and then dissipates in milliseconds. If you were to put 10,000 psi in the bore and keep the pressure that high while you leak checked the breech plug with a soap solution, it would be blowing bubbles like a two year old at a birthday party. 2) After the first shot, some powder particulate residue will pack into whatever little gas path is available and it makes a good seal itself. 3) The gas that gets past the not very efficient (even if very well done) face seal on the plug has to get past all the threads to make itself apparent. Each time the gas goes around a thread, it dissipates in pressure as it is forced to turn corners. In rocket engine turbo pumps we do this on purpose with something called a "labyrinth seal". With only an instantaneous driving pressure in the bore, and unless the breech plug fit is grossly poor, the gas will never make it out at the tang end of the plug.
All that said, my over wordy point is, do the best job you can fitting a plug, but it doesn't have to be perfect (for either performance or safety reasons). As Hungry Horse says the old time plugs worked fine and were far, far less precise than anything the barrel makers of today produce. Life is way too short to spend hours or days on a level of precision not required to do the job.
Just my thoughts
