Quality control in ammunition is
usually measured at the target. Notwithstanding the obviousness of such an approach, it is
complicated by combining the variables of shooter, gun, weather and ammunition. Ideally we
could test the quality of our ammunition separately, and in some manner other than a
return-to-battery benchrest gun fired in a wind-free tunnel, but we cant.
Still, the idea persists that we
should be able to ensure some level of quality control before venturing out to the range.
Weve improved our reloading dies and powder measures, added primer and flash hole
uniformers, developed means to measure seating depth and the distance from the lands to
the bullet at rest and, in general, raised the quality of bullets to an almost
unbelievable level. There is, however, one, almost intuitive, thing that far too few of us
do, and thats to measure the concentricity of our ammunition. We can do it at home,
without regard to gun or weather, and it can really make a difference on the target.
There are a number of tools on the
market for measuring concentricity, or the lack of it, and Sinclair International (2330
Wayne Haven St., Fort Wayne IN 46803) has recently brought out a new one.
Called the Sinclair Concentricity
Gauge, the tool utilizes a dial indicator as the measuring device and is offered with or
without the indicator, for those who already have one. The tool begins with an anodized
base plate measuring 3x6 1/2 inches and 1/2 inch thick. A permanently mounted post holds
an adapter that, in turn, holds the dial indicator. There is a longitudinal slot cut in
the base that accommodates the blocks used in positioning the cartridge case for
measurement. Each block is made of anodized aluminum and contains a pair of stainless
steel ball bearings that serve to position the case. Two blocks are used in concert and
one, the right one, has a vertical post that serves as a stop in positioning the case
longitudinally. In use the blocks are positioned in such a manner as to place the case
neck or bullet under the dial indicator probe and locked in place via handles that are
attached to threaded bolts. The case is then rotated by hand while resting on the ball
bearings and held against the stop.
Case neck concentricity measurements
should be taken at approximately the midpoint of the neck or equidistant from the case
mouth to where any neck turning, if done, stopped. Bullet concentricity measurements
should be taken on the parallel sides of the bullet, not on the ogive. Imperfections in
the case head can cause lateral
movement of the case that would cause the probe to move up and down on the ogive giving a
false reading. Any lack of concentricity, or run-out, can easily be read on the dial
An accessory came with my tool for
measuring variations in case neck wall thickness, although Sinclairs latest catalog
makes no mention of it. The company does, however, offer a separate hand-held tool for
such work called the Sinclair Case Neck Thickness/Variance Gage (why the company uses
gage in some instances and gauge in others is beyond me) that
operates in the same manner. The accessory for the concentricity gauge is a block that
mounts to the base in the same manner as the others, but only after the others are
removed. It contains a horizontal rod and is used in conjunction with RCBS Flash Hole Case
Pilot Stops. A pilot of the proper caliber is slid onto the rod and an unprimed, sized
case follows with the mouth of the case slid over the pilot and the end of the rod
entering the flash hole. The accessory is positioned so the case neck is under the
indicator probe and the case, thus supported, rotated while held against the pilot
shoulder or stop. Variances are noted on the dial, and a reading can be first taken
against the pilot to determine actual neck thickness. According to Sinclair, the tool is
capable of handling cartridges from the .22 Hornet to the .460 Weatherby. Additionally, it
appears that measurements can be made almost anywhere along the case body, if desired. The
entire tool is very well made and operates exactly as claimed.
The principle behind all this is
that maximum accuracy can best be attained if the centerline of the bullet of a chambered
round is precisely aligned with the centerline of the bore, and that case expansion when
the round is fired occurs in such a manner as to not disturb this alignment.
If anyone is wondering just how much
lack of concentricity is bad or what we can do about it, lets back up a minute.
First off, were generally talking about bottleneck cartridges fired in rifles or,
perhaps, single-shot pistols. Its not that any limitations of the tool preclude
testing concentricity on straight-walled cartridges, its just that the environment
in which these cartridges operate, including the guns, dies, bullets and case shape,
generally preclude the kind of tack-driving accuracy we can achieve in the bottleneck
cartridge environment. Why is a story for another time.
Some years ago the NRA, I believe,
published a study on concentricity in which bullet run-out was measured in front of the
case mouth of loaded ammunition. The study concluded that any run-out of less than .004
inch was undetectable on target. That was a long time ago and great strides have been made
in bullets, guns and dies, and today its no trick to achieve run-out of .001 to
.0015 inch and no reason to settle for less.
All such efforts begin with good
brass. Proper sizing die alignment, including a properly centered expander button, is
crucial as is bullet seating alignment. All this is measured with a concentricity gauge.
If I may make a few closing
observations: It seems to me the Sinclair Concentricity Gauge could be enhanced by the
addition of a series of holes along the back top of the base to hold the RCBS Pilot Stops.
Also that repeatability of block placement is desirable and that a witness mark on each
block and a series of such marks along the top front of the base would be a simple means
to accomplish this. Finally, since concentricity gauges need only be used with a new gun,
new dies or new or altered cases or after dies have been disassembled for cleaning, a
cover to protect the tool and its accessories would be well received. Fortunately, since
all the blocks can be secured to the base and still remain within the footprint of the
base, such a cover could be simple to make.
Concentricity gauges are among the tools that,
while used infrequently, can greatly enhance our knowledge of our guns, our dies and our
handloading techniques. Sinclair makes a good one. - R.H. VanDenburg, Jr.