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What a Chamber Throat Is

By: Mike Bellm
     In normal applications, the throat is nothing more, nor less than an area in the barrel immediately in front of the chamber neck that allows the bullet in a loaded round to project out into the rifled part of the barrel. For this to be possible, the rifling must be cut away. Thus, the throat is actually the groove diameter of the barrel with the rifling cut away.

     For best accuracy, the groove diameter of the barrel should be enlarged very little if at all, and then only enough to allow a bullet to fit in it. This insures that the shank of the bullet is supported and guided as it is forced into the rifling. To do this, the area in front of the chamber neck must be a cylinder, not a cone. If it is a cone, then only the loose fit of the case neck in the chamber neck gives any alignment and support of the bullet as it enters the rifled part.

     With at least several thousandths of movement possible in any random direction at the case neck, even if the bullet is centered in the rifling at its nose, as in seating bullets out to the rifling, the base of the bullet can still deviate in any direction out of alignment with the bore, and thus enters the rifling at an angle, canted, cockeyed, or whatever term you relate to that means it did not go in straight.

     This throws the bullet out of balance and accounts for much of the inaccuracy in all barrels. This is not my dreamed up hypothesis. In addition to my own observations, it is also confirmed by a nationally prominent scientist and shooter, Harold Vaughn, in his highly technical book "Rifle Accuracy Facts." In his book, Vaughn went so far as to measure the actual amount of bullet dispersion at the target caused by specific angles of "cant" of the bullet as it enters the rifling. The straighter the bullet enters the rifling, the greater its potential for hitting in the same place as the bullets fired before and after it.

     The taper cut on the ends of the rifling, also known as the "leade," helps center the bullet as it enters the rifling, and the longer this angle is (presumably within limits), generally the better the leade guides the bullet straight into the rifling.

     Regarding the case neck in its role of aligning the bullet with the bore, the forces involved pushing the bullet into the rifling are much greater than a thin little brass tube, the case neck, can offset. Only in tight necked chambers with case necks outside turned to give a loaded case neck diameter that is a tight fit in the chamber neck can the case neck do anything to positively align the bullet with the bore. But conversely, a closely fitted steel cylinder, the throat, in front of the chamber neck CAN offset the effects of misalignment in the case neck... and the chamber itself to a large degree, for that matter.

     The throat is where its at. Yet this is probably one of the very least talked about, least understood aspects of all we think we know about rifled firearms. When was the last time you read anything about throat DIAMETER in any popular shooting publication anywhere, IF EVER. Everyone talks about overall cartridge length and seating depth, but no one is talking about the actual DIAMETER of the throat, let alone some of the design issues I discuss on this site.

Cutting a minimum diameter throat with a long leade angle and aligned with the bore will do more to improve the accuracy of TC barrels than any other single factor.

- Mike Bellm

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This article reprinted with permission of Mike Bellm and

Warning: All technical data mentioned, especially handloading and bullet casting, reflect the limited experience of individuals using specific tools, products, equipment and components under specific conditions and circumstances not necessarily reported in the article or on this web site and over which The Los Angeles Silhouette Club (LASC), this web site or the author has no control. The above has no control over the condition of your firearms or your methods, components, tools, techniques or circumstances and disclaims all and any responsibility for any person using any data mentioned. Always consult recognized reloading manuals.