The Voyeur's Guide to Barrel Chambering - Part 2

The Voyeur's Guide to Barrel Chambering - Part 2
by Germán A. Salazar and John Lowther

As we take up the second installment of the series, our attention turns to threading that lovely barrel tenon that we cut in the previous installment. With the exception of some rimfire actions and perhaps an odd centerfire, basically all barrels are threaded into the receiver. The quality of threading has a significant effect on the ultimate accuracy potential of the rifle; the threads must be concentric to the bore and a good fit to the receiver's threads. The concentricity is taken care of by performing all machining operations on the breech end of the barrel without disturbing the original setup in which the bore was centered in the lathe.

Photo 9 - Cutting the relief groove on the tenon
Clean-up and Groove
The first step John performed was a clean-up cut across the breech face of the barrel. This removed the stampings and gave a clean working surface for some future operations.

The next step (Photo 9) was cutting the relief groove at the base of the tenon. The purpose of the groove is to give the machinist a little bit of breathing room as the lathe advances the thread cutting tool automatically but the machinist must manually stop the advance.

Without the groove, he has to pull the cutter out of engagement at the exact moment that it's about to crash into the shoulder (and break) on each of a dozen or more passes required to cut the threads to full depth. For this reason, some call the relief groove the "chicken groove" as it takes a mighty brave (and skilled) machinist to thread the tenon without one. The presence or absence of the relief groove does not affect accuracy.

The relief groove is cut with the tool angled slightly away from the shoulder to ensure that the shoulder surface isn't altered. If you look carefully, you'll see that the groove cut actually doesn't extend all the way to the shoulder surface.

Photo 10 - Squaring the thread cutting tool to the tenon
Threading Basics
At this point, the cutting tool used to cut the relief groove is removed from the tool post and a thread cutting tool is inserted. The thread cutter must be squared to the tenon as the tool post rotates and can bring a tool to bear at any angle. Photo 10 shows the use of a simple alignment tool to set up the cutter at a 90 degree angle to the tenon.

The cutter is for a 60 degree thread angle, standard for almost all receivers. Some older rifles such as the M1 Garand use a square thread, you can research thread types someday and entertain yourself. For our present purposes, we'll leave that topic aside.

Photo 11 - Applying machinist's blue to the tenon
The final step before we begin to cut the threads is applying a coat of machinist's blue to the tenon. This will allow us to make a very light initial pass with the cutter to ensure that the lathe is properly set to cut the desired thread pitch. In this case, for the BAT receiver, the correct pitch is 18 tpi (threads per inch). In the event that the lathe were improperly set, it could be re-set to the correct pitch and the threads cut with no interference from the initial super light pass.

Photo 12 - Slow but certain progress cutting the thread
Cutting and Checking the Thread
Once that first super light pass was made and the 18 tpi thread pitch confirmed, John began to make his actual cutting passes. Just like cutting the tenon itself, threading the tenon is slow and careful work with each pass removing only a tiny amount of steel.

The lathe picks up the thread at the same starting point on each pass and moves the cutter ahead automatically. The machinist watches his indicators, whether mechanical or digital, and pulls the tool away from the barrel when it reaches the end of the travel (either the groove or the shoulder as the case may be).

Photo 13 - Trial fitting the thread
Once the thread reaches a depth close to the the final, the trial fitting begins. If the thread depth is too shallow, the receiver won't screw on because the threads in the receiver can't engage those in the barrel. Slowly, over the course of several more passes, the fit goes from zero, to tight, to snug, to just right. This step calls for a great deal of care and a good touch by the machinist as it would be easy to gall the barrel thread and receiver thread during the trial fitting.

Photo 14 - Final thread fit to the receiver
Finishing Up
Eventually, the thread is cut deep enough and the tenon will screw into the receiver with the right feel (clearance). At this point you will probably discover that the tenon is a bit long and it runs into the locking lug abutment in the receiver before the barrel shoulder makes contact with the face of the receiver. A feeler gauge can be used to measure the gap between the shoulder and the receiver face, then that amount is machined from the back of the barrel. The goal is to achieve solid contact between the shoulder and the receiver without creating a big gap inside at the locking lug abutment.

Photo 15 - Final threading on the barrel tenon
So there you have it, a nice looking, good fitting thread on a tenon of the appropriate length for the receiver.

The slightly rough look of the threads in the picture is from the anti-sieze compound on them, the threads themselves are quite smooth.

Click here for Part 3

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