Tight Chambers (Why some rifles are more accurate.)


A "tight chamber" is only part of the story.

      Most shooters know that accurate rifles always seem to have a "tight chamber".   However, there is a great deal more to it than that.   The process to cut a chamber perfectly concentric with the bore is extremely time consuming, and not every machinist (even good ones) can do this job accurately enough.   I am talking about cutting the chamber concentic to the bore within .0001" (that's one ten thousandth of an inch.)   It is only after the final cutting operation that the success of this operation will be known.   If production rifles were consistently made to these close tolerances, the price tag would be shocking.

      To start off with, a top notch gunsmith will always supply chamber specifications to the manufacturer for his chamber reamers.   The best chamber dimensions are usually known by gunsmiths that specialize in building accurate rifles.   Most shooters (and some gunsmiths) are surprised to find that a super tight chamber body diameter doesn't do anything for accuracy.   In fact, if the chamber body diameter is too tight, it can make a standard full length resizing die ineffective.   This often causes a "fail to chamber" symptom, and it is one of the most common "bonus problems" that handloaders encounter with custom "tight" chambers.

      However, the chamber clearance at the neck should be minimal (somewhere under .002") diameter depending on your need for extreme accuracy and if you intend to turn your case necks.   This requires your gunsmith to know the exact thickness of your case necks and the particular bullet you intend to use.   He also needs to know if you intend to turn your case necks - or not.   The diameter of the chamber "leade" should also be tight.   The chamber leade on my best shooting rifle chambers have .0005" (half a thousandth of an inch) clearance over actual bullet diameter.   The rifle shooter should know in advance which bullet he intends to shoot, because the entrance to the throat and freebore should be shaped for that particular bullet.

      The tools and techniques required for making precision handloads are quite simple.   Some shooters are surprised to find that neck turning provides little or no accuracy benefit in some chambers.   The handloader should pay close attention to his actual chamber measurements - especially the neck diameter.   I have found that most chambers with a large diameter neck clearance will shoot even worse if case necks get turned.   This just adds "extra" clearance to the case neck, and the case neck (by itself) provides no firm support to keep a bullet aligned as it is fired.

      The average shooter can easily determine the exact neck clearance of his chamber by measuring the diameter of one of his loaded rounds (at the neck) . . . . then deduct that diameter from one of his fired cases.   It is especially important with any tight neck chamber, to measure the neck clearance of your handloads when switching to a different brand (or a different lot) of brass.

      Other important considerations include knowing the maximum over-all length (OAL) for your chamber.   This is usually limited by your magazine length, if your rifle uses one.   I specifically designed the chamber on my most accurate 300 Win. Mag. rifle to shoot the 220 gr. Sierra MatchKing bullet.   My chamber allows the rear bearing surface of the bullet to line up with the rear of the case neck.   That leaves almost .300" of remaining bearing surface of the bullet extending from the case.   The long extended bullet fits snug into the leade of the chamber within .0005" (that's half a thousandth of an inch).   This long extended surface centers the bullet with a firmly "supported" fit that delivers the best long range accuracy.



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Larry A. Willis,   Innovative Technologies
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