The Voyeur's Guide to Barrel Chambering - Part 3

The Voyeur's Guide to Barrel Chambering - Part 3
by Germán A. Salazar and John Lowther

Photo 16 - John Lowther
 As we continue our series, the breech end of the barrel remains the focus of our attention. Now that we have a threaded, properly dimensioned and well centered tenon on the barrel, it's time to prepare the end of that tenon to mate with the bolt.

There are various breech face styles, varying by action manufacturer and sometimes by the rifle's intended purpose. The Remington style with a recessed face that envelopes the bolt nose is very common and it provides excellent gas handling and safety in the event of a case rupture. However, feeding is not as smooth as with a coned breech face such as the Springfield 1903 and the original Winchester Model 70 used. A few actions call for a flat breech face, the RPA Quadlock is one example of this.

This project involves a BAT 3 lug action with a coned breech face; accordingly, our next step is cutting the cone and finalizing the tenon length to properly fit this action.

Photo 17 - Cutter centered and ready

Cutting the coned breech face, like every other operation involves careful measurement and many small cuts until the final dimensions are reached. In Photo 17 we see the proper cutter, aligned with the bore, ready to begin.

Photo 18 - Cutting the cone

The first cut removes a tiny amount of metal near the bore. The next cut begins a little deeper and thus comes out a little further along the breech face.

Photo 18 shows the process after five or six passes. The cone angle is set by the angle of the cutter and remains fixed throughout the process.

There are coned reamers made now (think of a large deburring tool) but this is the old way and can be adapted to any cone angle. The BAT, for instance, calls for a 25 degree cone angle, whereas most other actions use a 30 degree cone.

Photo 19 - Measuring the shoulder gap
 Just as when we were establishing the basic length of the tenon earlier, the feeler gauges come back into play as we establish the final length of the tenon.

The tenon must be short enough to allow the receiver to screw on all the way without running into the locking lug abutments. However it must also be long enough to provide a close fit between the bolt nose and the cone, allowing just enough clearance to ensure proper functioning in all conditions (heat, cold, dust, etc.).

With the bolt in the receiver, the action is screwed on to the tenon until it stops. The resulting gap at the shoulder is measured and that amount of material is removed with additional passes on the cone. Eventually, the gap is zero and the shoulder comes into full contact.

Photo 20 - Finished and polished cone
  Of course, at that point, the bolt nose to cone clearance is also zero, so additional passes are made to push the cone deeper, creating the desired clearance. Too much clearance between the bolt nose and the cone is undesirable as it creates a larger rearward escape path for high pressure gas in the event of a case failure. Excessive clearance also increases the possibility of such failure by leaving more of the case hanging out of the back of the chamber without support. We settled on 0.015" as our nose-to-cone gap based on the very dusty conditions in which the rifle will be used.

The finished cone should be smooth for good feeding characteristics. A little polishing with fine grit paper may be necessary once the cutting is done. No need to make a mirror out of it, but it shouldn't catch your fingernail as you run across the surface.

Photo 21 - Visual check of bolt nose fit to the cone

Photo 22 - Front view of finished cone
 Photo 21 shows the nose of the bolt against the cone, this isn't part of the machining process, we're just admiring the matching angles. Photo 22 is just another view of the finished cone.

Photo 23 - Action screwed onto the finished breech end of the barrel, but no chamber yet
 The final result is that the action screws on to the barrel and actually looks like it's ready to fire. Of course, we haven't cut a chamber yet, so appearances are indeed deceptive in this case. Tangible progress, nonetheless, and quite satisfying to reach this stage.

 Click here for Part 4


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