The Voyeur's Guide to Barrel Chambering - Part 4
by Germán A. Salazar and John Lowther
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| Photo 24 - Chamber reamer with removable pilot bushing |
Reamer
Photo 24 shows the reamer, which will be used in this project. The reamer has a pilot bushing at the front; this is removable and the gunsmith has a large selection of bushings to exactly match the bore dimension of the barrel being chambered. This is a trial fit process that requires the gunsmith's judgment as to a fit that is just tight enough to guide the reamer without play, but not so tight as to damage the lands.
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| Photo 25 - Reamer entering the bore |
The barrel will turn and the reamer will remain stationary (in a rotational sense) to make the cut. The reamer is not locked down rigidly at the rear; it is held in a floating manner, and pushed into the turning barrel slowly. This ensures that the reamer follows its pilot bushing into the bore which itself was centered in the headstock in our first operation.
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| Photo 26 - Reamer cutting the chamber |
Just as with each cut that we've seen before, the chamber is cut with many, many passes. The reamer is cutting a lot of metal out of the barrel and the flutes load up with chips quickly. If they aren't removed regularly, they can get caught between a cutting surface and the chamber wall and leave a deep scratch which will have a very adverse effect on extraction. Consequently, the reamer is run in about 0.050", removed, cleaned, re-lubed and re-inserted for another similar cut. This happens dozens of times before we're close enough to slow the process down even more.
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| Photo 27 - Inserting the go gauge |
When the reamer has reached a depth that is close to final, but still some safe distance away, we screw the action back on, slip in a "go gauge", close the bolt and measure the shoulder gap as show in Photo 19 (Part 3).
Photo 27 shows the receiver all the way against the barrel, but in order to close the bolt at this stage, it will have to be backed off a bit.
The go gauge is a steel duplicate of a minimum size cartridge without the neck area. The finished chamber should allow the bolt to close on the go gauge. A no-go gauge is the same thing, but 0.004" longer; the finished chamber should not allow the bolt to close on the no-go gauge.
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| Photo 28 - Reamer, PTG's Lambeth reamer micrometer and the reamer holder |
With the shoulder gap measured with the feeler gauge, we know how much deeper the reamer needs to go in order to just barely let the bolt close on the go gauge. Now the trick is to bring the reamer in just that amount. John uses a micrometer type tool designed by Nat Lambeth and sold by Pacific Tool & Gauge which allows him to create a positive stop at the desired depth; that's what you've seen attached to the reamer in the previous photos.
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| Photo 29 - PTG reamer micrometer against the breech |
Photo 29 shows the micrometer stop set right against the barrel, this would allow no further travel of the reamer. From there, it is backed off by the desired depth of the next cut. For instance, if we calculated that the reamer needs to go in 0.050" more, that might be divided into three or four more cuts, so the micrometer might be set to allow 0.020" travel on the next pass, then 0.015", then 0.010" then 0.005" - that's just an example, not necessarily the actual numbers used. The point is simply that you really sneak in on the last little bit and this tool allows you to do that with great precision and repeatability.
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| Photo 30 - John borescopes the chamber and throat |
After each of these final stage cuts, the action is installed on the barrel and the shoulder gap measured. Once we hit the point where the bolt closed on the go gauge with the shoulder gap at zero, we inserted the no-go gauge and measured the shoulder gap; it was 0.004", as expected. Ultimately, the barrel will be torqued on to the receiver and there will be a bit more rotation than with the hand tightening used during the checking process. That rotation will reduce headspace a little bit and it is possible that the chamber would be a bit short. Therefore, at this point we made one last cut of 0.002" to allow for the additional barrel rotation on final installation while remaining within the allowable range. As it turned out, this was a great call, with fired brass being just where I wanted it and allowing easy resizing. John cleaned out the bore and borescoped it to make sure that the chamber and throat were free of scratches or burrs.
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| Photo 31 - Turning the shank to final diameter |
With threading and chambering complete, the barrel was almost ready to come out of the lathe, however, one last operation remained on this end. That last step is to turn the shank down to match the old barrel to ensure that the barrel will remain free-floating in the stock. A few light passes of the cutting tool and some light polishing took care of that. Finally, all of the breech-end work was done; the barrel came out and was flipped end for end on the lathe with attention turning to the muzzle next.
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| Photo 32 - Ready to work on the muzzle end |
