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Co-Ax Throating . . . What is it?

By: Mike Bellm

In short, it is a term for what is presumed to be present in all barrels, but isn't.

     When you buy a barrel or have one re-chambered you assume the hole cut into the barrel for the cartridge and bullet protruding from it is centered with the lands and grooves of the barrel. Sadly, this is not the case in far too many instances. Using traditional chambering methods commonly used, seldom is the critical throat area of the barrel aligned with the bore as well as it could be.

     Knowing what to look for, evidence of misalignment of the throat can readily be seen with the naked eye by simply looking into the barrel and examining the circumference of the point where the throat section of the reamer stopped. In most instances there will be a connecting line between the ends of the rifling in the groove area. Look closely and you will see that this line is more pronounced in part of its circumference and fainter opposite that point. This indicates the throat section of the reamer cut deeper in one part of the groove diameter and is not perfectly in line with the bore.

     This offset of the throat from the centerline of the bore causes bullets to not go into the rifling straight, and is a signficant reason why some barrels shoot better than others.

     Further evidence is easily verified by bore scope inspection where the offset is VERY clearly seen. Plus, if the barrel is set up in a lathe with the bore dialed in "true," running an indicator at the mouth of the chamber clearly shows that many barrels typically have .010-.020" run-out, and are thus also out of alignment with the bore at an angle to it. Better custom barrels show much less run-out, but such run-out is almost the rule among factory barrels with somewhat rare exceptions.

     The above is commonly dismissed out of hand by folks with closed minds who won't look any further than the sights. But if one can see well enough to shoot well, with an open mind, the eye can easily see the error built into barrels by the one cutting the chamber.

     Examination of the throats I cut will reveal a much different appearance than those cut by most other sources. The difference can be quite pronounced.

     There are other factors of course that effect accuracy, but in any given barrel, for optimum accuracy, the bullet must have a straight shot into the bore/groove area of the barrel.

     Another HIGHLY significant factor is the diameter of the throat, which on nearly all commercially ground reamers I have had is too large.

     Throat DIAMETER should be as closely matched to bullet diameter as is functionally possible, plus the throat should have as long leade angle as is practical.

     This is all aimed at the objective of positively holding the bullet in alignment with the bore as it engraves into the rifling.

     A thin brass tube, the case neck, sitting inside a hole with at least several thousandths of clearance, the chamber neck, and also usually angled off line to some degree with the bore simply cannot provide much alignment of the bullet to the bore.

     With the bore dialed in as true as its straightness allows, I throat all barrels separately with minimum diameter throat reamers having long leade angles.

     For lack of a better term to describe it, this is what I call... Co-Ax Throating.

     Its sole purpose is to help you easily get excellent accuracy from even mediocre quality barrels and top accuracy from high quality custom barrels, whether by re-chambering an existing barrel to a longer chamber with a new Co-Ax Throat or chambering a new custom barrel.

- Mike Bellm

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This article reprinted with permission of Mike Bellm and
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