Sinclair International’s Concentricity Gauge

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 can’t.

Still, the idea persists that we should be able to ensure some level of quality control before venturing out to the range. We’ve 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 that’s 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 indicator.

An accessory came with my tool for measuring variations in case neck wall thickness, although Sinclair’s 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, let’s back up a minute. First off, we’re generally talking about bottleneck cartridges fired in rifles or, perhaps, single-shot pistols. It’s not that any limitations of the tool preclude testing concentricity on straight-walled cartridges, it’s 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 it’s 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.