The Voyeur's Guide to Barrel Chambering - Part 1

The Voyeur's Guide to Barrel Chambering - Part 1
by Germán A. Salazar and John Lowther

A voyeur's guide to barrel chambering? Well, yes - this series isn't intended for anyone who owns a lathe; instead it is for those of us who send an action off to get a new barrel installed. Those who have the equipment know what to do and how to do it and I have nothing to teach them. On the other hand, if you've ever wondered just what goes into barrel fitting, this is it. We could have titled it: Everything You Always Wanted to Know About Barrel Chambering But Were Afraid to Ask - but that's kind of long for the index page.

The basic steps covered are threading, chambering and crowning the barrel blank. Each machinist who performs these operations has his preferences from among many possible ways to accomplish these tasks; what we'll show is John's way. If your man has a different approach, that's great, it's what works for him and this isn't intended to have you ask him to change his ways or to say that the way we show is the best or the only way; all it is, is one way - John's way.


Photo 1 - BAT action with new barrel blank (above) and old barrel (below)

John generously volunteered his time, effort and machinery for this series, and I must reiterate what we've mentioned before: John is not a professional gunsmith, he does this for his own entertainment and does not accept outside work. The division of labor for these articles is simple: John does all the hard work, I take pictures, type the text based on John's descriptions of each process and generally try to stay out of the way while the lathe is turning and the chips are flying. As always, just click on any picture to enlarge it for more detail.

The project at hand is a very typical one: install a new barrel chambered in .308 Winchester on a modern target action. In this case, the action is a BAT with a coned bolt face. The processes shown and described in the series are the same for just about any action, where details differ we will try to point that out.


Photo 2 - Breech end of new Krieger barrel blank

When you buy a barrel blank, there is quite a bit of information stamped on the breech end, let's take a look at it. First, the obvious manufacturer identification, this is a Krieger. At the top is the barrel's serial number, you'll need this if there is something wrong with the barrel, so write it down. Below you see the letters NF, I don't know what that is, maybe the machine operator's initials, but I'm just guessing. On the left you see the bore and groove dimensions, this one has a 0.300" bore diameter and a 0.308" groove diameter. These are standard dimensions for a .30 caliber barrel, but Krieger (and most manufacturers) offers some small variations on those dimensions. On the right is the all important rate of twist, in this case the 11 indicates that the rifling has a 1:11" rate of twist. Just below that, the SS confirms that the barrel is made of stainless steel as opposed to chrome moly (CM).


Photo 3 - Barrel in the lathe
The blank is inserted into the lathe with the breech end in the headstock and the muzzle end supported by the spider.


Photo 4 - Checking and minimizing runout at the muzzle end
The dial indicator on the muzzle end is used to watch the runout of the muzzle end of the barrel. Using the four adjustment screws in the spider (look at Photo 3, just over John's thumb) the muzzle end is centered up to minimize wobble. However, the real work of centering things up will take place on the breech end; this is just a preliminary operation.


Photo 5 - Indicating the breech end of the barrel
How much of the breech end to have sticking out of the headstock is determined by the length of the barrel tenon (the threaded part) as well as any amount you may want to cut off. The key is to keep the unsupported length of the barrel to a minimum, this will help the cutting tools work better.

The real work of indicating the barrel happens now. A long probe goes into the barrel, resting on the lands and grooves at least one inch in. The chuck is rotated by hand with an eye on the indicator. The chuck's jaws are adjusted to center the barrel perfectly in relation to the bore. This process can take quite a while, depending on the type of chuck used and the machinist's level of experience, but it is critical to a good final result. The reason that the probe is inserted at least one inch is to get it past the slight irregularities that exist right at the end of the bore. These are caused by the lapping tool reversing direction there during the barrel manufacturer's final lapping process.


Photo 6 - Measuring the thread diameter on the old barrel
If a barrel that was previously used on the rifle is available (and assuming it was done properly), it can be used to get a basic idea of the outside diameter of the threads. Although the final dimension will be based on trial fitting and a certain degree of feel (based on the machinist's experience) this can save a bit of time initially.





Photo 7 - First cutting pass begins
It's finally time to make some chips fly! With cutting oil on the barrel to keep everything cool and smooth, the cutting bit begins to bite into the barrel tenon.

The cutting tool removes a few thousandths of an inch of metal at most, so many passes are required to reduce the tenon diameter to the desired end size.



Photo 8 - Barrel tenon at final diameter before threading


After many passes of the cutting tool, the tenon is at the desired diameter and close to the final length. Reducing the diameter to 1.0605" from the original 1.250" took quite some time and effort.

This cut also establishes the barrel shoulder which will mate up to the receiver shoulder. All of the time and effort spent indicating the barrel to have the bore perfectly true to the headstock pays off here in producing a tenon that is parallel to the bore and a shoulder that is perpendicular to the bore.

Click here for Part 2
 

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