Sinclair International’s G3 Rimfire Thickness Gauge

For accuracy-conscious handloaders of centerfire ammunition, quality control is part of the reason we go to all the effort. Chief among these controls is the ability to match each case to the chambers - at least in loading for most rifles and single-shot pistols - even after some sizing of the case. The rimfire shooters among us must forego all that, both the effort and pleasure of reloading and the matching of ammunition to chamber.

Still, rimfire enthusiasts are not without any means of ammunition quality control. One of the newest of these tools is the G3 Rimfire Thickness Gauge from Sinclair International of Fort Wayne, Indiana.

The name refers to the tool’s ability to measure the thickness of the cartridge rim. There are two reasons for doing this. First is that rimfire cartridges headspace on the rim and some guns seem to be more accurate with a particular thickness. Second is that the priming compound in a rimfire cartridge is distributed within the rim. Differing rim thicknesses could hold differing amounts of the compound and, in turn, effect ignition and consequently, accuracy.

There are several tools available that are designed to measure rim thickness. None seem to be any easier to use than this one or any more accurate or any less expensive. This latter, in part, because it is designed to be used in conjunction with another tool it is assumed we already have - a caliper. Fair assumption, for if you’re reading this, in this magazine, you’ve most likely accumulated a few tools, including a caliper.

The Rimfire Thickness Gauge is made of aluminum and is about 1 7/16 inches long. It vaguely resembles the neck, shoulder and beginning of the body of a cartridge case. A cut in one end allows the tool to be affixed to the movable jaw of a caliper and secured by a screw. The other end is open to receive a cartridge. The tool’s use is restricted to standard .22 fodder: .22 Short, Long and Long Rifle and even CB and BB caps, if you’re interested. It will not accept .22 Winchester Magnum ammunition or the older .22 WRF or .22 Remington Special. Attached to the tool is a lanyard, designed to be slipped around the neck when the tool is being used. Displaying the brilliance of simplicity, the lanyard can prevent the tool from being dropped. As all rimfire fans know, it is percussion that fires the round, whether in the chamber or in a tool such as this dropped on a hard surface. As the instructions note: “The neck you save may be your own!!” Or, I might add, some one else’s. Regardless, the gauge is simple and safe to use, and made even safer with the lanyard.

According to Sinclair’s literature, the tool is designed to be used with a digital or dial caliper. Actually it can be used with a vernier caliper as well. The difference is in ease of use and, perhaps, a slight loss of accuracy when a vernier style is employed. When the gauge is affixed to the movable jaw of a digital or dial caliper and the jaws closed on the empty tool, the caliper can generally be zeroed, allowing measurements taken with a cartridge in place to be absolute. With a vernier caliper, no zeroing can take place. Instead, the “empty” reading must be recorded as a base and each subsequent measurement with a cartridge in place recorded and the difference determined to arrive at an absolute thickness. In practice, this can often be done in one’s head, or ignored, simply using the measurement with a cartridge in place. Those with the same measurement get sorted, and fired, together.

Using the Rimfire Thickness Gauge is dirt simple. Justifying its use is another matter. For most of us, with our over-the-counter guns with their over-the-counter chambers, shooting whatever .22 ammunition is on hand is just fine. We get our offhand practice, plinking or small game hunting and we’re happy. There’s certainly nothing wrong with that. Some of us, though, go to the trouble of determining which brand of ammunition seems to be the most accurate in our gun and only use that brand. This thoroughly practical approach is sometimes expanded upon to find a single lot of one brand that is most accurate

and then buy as much of it as we can. We then reserve the use of that ammunition for that particular gun. I recently took this approach with a new .22 pistol for which I had high hopes. It took awhile, but I finally found a particular brand and lot of ammunition that met my demands. I quickly bought a case of it.

Some of us take all this even farther and enter the competitive arena. Bullseye, silhouette and bench-    rest competitions abound for the rimfire addict. Benchrest shooters compete under BR-50 International sanction. No off-the-shelf stuff here. Custom rifles with custom barrels and custom, match chambers are the norm. Ammunition is often tested, weighed, measured and sorted until each round in each group is as alike as is possible. Measuring rim thickness is part of the game.

Others who might be attracted to tools such as these are folks like me who would like to quantify those qualities that make some ammunition more accurate than others. For example, in comparing a half-dozen brands on hand recently, I found several things of interest. The most accurate ammunition was the most expensive, although no where near the $10 to $12 per box being charged for top-of-the-line ammo these days. It had the thinnest rim thickness, averaging .034 to .035 inch compared to the .037 to .040 inch of others; it had the smallest extreme spread in rim thickness, .002 inch compared to the .003 to .004 inch of others. The most accurate ammunition also had the smallest extreme spread in cartridge weight, .3 grain over a box of 50, with the vast majority within .2 grain, compared to .4 to .5 grain differences in other brands. I also found the most accurate ammunition to be very slightly larger in diameter, perhaps offering a better fit in over-the-counter chambers.