Good Morning.
Before I go any further, I want to talk a little bit about the drilling of the chamber to rough it out. If a drill bit is properly sharpened it will want to follow a hole and will cut to center. If the hole is offset, this will, of course, cause the drill bit to have to bend in order to follow the bore and due to side loading, the drill will cut oversize at the tip as it is fed further into the work. If the bore starts out concentric and running true but is crooked further in, the drill will begin to struggle to cut to the center of the bore and the rear of the hole will become oval. If this condition (a crooked bore) exists to a significant extent it will be difficult, though not impossible, to produce a straight, co-axial chamber. Our barrel though, is nice and straight (ish) so the drilling is straightforward. Don't drill too slowly; push it a bit. A drill bit performs best under a bit of a load so don't baby it. Now, to the reaming.
Holding the reamer.
There are numerous ways of holding and guiding the chambering reamer. There various floating holders on the market. Some like to hold the reamer in a chuck. The most common method is to hold the reamer with a tap wrench or a wrench made specifically for the purpose and support the reamer on the tail stock center. A variation on this is to use a wrench but, rather than support the reamer with a center, use a "pusher" which allows the rear of the reamer to float. There are more variations within this sub-set regarding the design of the pusher and the wrench but I'm not going to worry about this now. Like I said, the most common is probably the use of a tap wrench while supporting the reamer on the center and that is the method I'll use here. Some people like to use a wrench which will clear the toolpost or carriage if they let go of it and the reamer is diven with hand pressure only. Others like to be able to let the wrench rest on the toolpost. I like to rest the wrench on the tool post early on in the process then hold it with my hand toward the end. When the wrench is supported on the toolpost there is undoubtedly some side pressure and, if the reamer grabs, it's likely to break. I hope to avoid this.
For cutting lubricant, I have always used Rigid thread cutting oil. Dark or Nu-clear; your choice (I like the Nu-Clear). I am beginning to believe it doesn't make a whole lot of difference as long as the oil can stand up to the pressure at the cutting edge. I had a friend ( a hobby gunsmith who is also a machinist by trade) tell me motor oil would probably work as well. I tried some 30 weight and it did indeed work just fine. Nonetheless, I like the Rigid product. I have used through-oiling systems-where the oil is pumped through the barrel and past the reamer- and they work well but I'm just as happy with the oil can method so that's what I'll use on this barrel.
To start, I blow out all of the chips which the drilling operation might have left behind by directing the air into the muzzle end of the barrel via a long tube attached to the air nozzle. Before I start reaming, I have a good look to make sure no chips remain. I squirt a glop of oil into the chamber-to-be and anoint the reamer as well. I place the reamer on the tailstock center and push the tailstock ahead while directing the reamer into the hole. When tghe shoulder of the reamer is nearly contacting the mouth of the chamber, I lock the tailstock and begin feeding with the handwheel. At this stage of the game, I'm not too worried about finish or anything so I feed the reamer relatively quickly and I'll probably feed in about a half inch before I stop. When I feel I've gone far enough, I unlock the tailstock lever and drag the tailstock back while -and this is important- making sure to hold the reamer tight against the center. Now, I blow out the barrel again and blow the chips off the reamer. I then runa fingernail along each cutting edge to make sure no chip has welded itself to the reamer. If I feel a bit of metal, I use a piece of triangular stone against the face of the flute to remove the metal then repeat the whole process. Once the neck starts to cut, I quit feeding in so far with each cut and may only cut in .125" or so. Once the shoulder starts to cut, I stop and clean the chamber out well and measure the depth using the depth mike and a "go" gauge. Don't tip the gauge and do double check your measurement. I will have roughed to within about 1/10 inch of the finished length so I now feed in only about .075" and I feed more slowly; especially for the last few thou. Then it's clean and measure again. I should have somewhere around .025" to go (let's pretend I'm dead on and that's exactly what it measures. I check the reamer very carefully and, again, stone each face lightly to make sure the edges are clean. Then squirt in another glop, oul the reamer, and take the final cut. This time, the reamer is fed in quite slowly and, for the last five thou, very, very, slowly with very little pressure. When the tailstock wheel reads .025", I let the reamer dwell for one turn then pull out. Another blow and it's done. I wipe out the chamber and have a look and it is, of course, perfect and free of tool marks. I use a de-burring tool or a case neck deburring tool to break the edge of the chamber and produce a bevel or a radius, depending upon the tool. I then use a drill with a slotted piece of aluminum rod into which I have placed a piece of fine Scotch-brite then, with the lathe running, run the Scotch-Brite in and out three or four times. This gives the chamber a slight polish but really it just cleans it out more than anything. There should be no significant toolmarks which need to be polished out.
Now the barrel is reversed so the muzzle end is at the tailstock and the steady is set to run just behind where the barrel is to be finished. The end is parted off and the barrel is crowned with a crowning tool or with a little boring bar dependent upon which type of crown you want to cut. The finishing touch is to break the edge of the crown using the piloted center reamer. When I do this, I make very certain the bore is clean and the pilot is absolutely clean; any microscopic chip can visibly mark the bore and we don't want that. Run the lathe at 240 (give or take) and just hold the reamer with your fingers while it is supported by the tailstock center. The machine work is now complete.
This is, more or less, the method which was taught to most gunsmiths fifty years ago and which is still use, with some variations, by many shops, including custom barrel makers, today. Each step is dependent upon the proper performance of the step previous. If the initial centers were cut perfectly concentric to the bore, subsequent operations should also be concentric. There are, of course, potential problems which may compromise the job and I'll discuss some of these later. There are reasons for using this technique. These reason include the following:
1. A spindle hole or a lathe which is too small.
2. A headstock which is too long. I once, out of necessity, had to fit and chamber some barrels on a 24 x 120 lathe with a headstock which was better than three feet long. There was no way to support the muzzle end if the barrel was held in the four jaw chuck (well, there was a way but it meant making fixtures which I didn't want to do). Supporting the reamer on the center while trying to gently push it into the chamber with a 150 pound tailstock was a real treat. Sometimes, bigger isn't better.
3. A worn lathe on which the ways are worn down near the headstock.
4. Expedience. This method is fast. If I was doing the job described- a Ruger 77 with a pre-contoured barrel. I could do this job in an hour and a half from the time I picked the rifle out of the rack until I put it back in. This assumes a stainless barrel, of course, with no bluing but would include polishing, bead blasting, and marking. This largely why barrel makers often use this method or some variation of it. The method is viable for precision work with some refinements and with verification of the set-up for each step. I'll go into this further later on and describe some questionable practices which have, nonetheless, been used successfully (sometimes) by accomplished 'smiths. I'll also get around to discussing solid pilot reamers and maybe a little on reamers in general. I've got a busy day ahead of me though so it won't be right away. Later.
