G3 Rimfire Rim Thickness Tool and Base-to-Ogive Length Gauge

Gerry Gereg makes two precision tools that let rimfire shooters pre-sort their ammo for improved accuracy. The first tool, the G3 Rimfire Thickness Gauge, lets you sort rimfire ammo by rim thickness. This tool clamps to the jaws of your calipers and is very simple to use. Just slide a cartridge into the gauge and slide the jaws closed. With mid-grade rimfire ammo you’ll see variances of up to .006″ in rim thickness. High-end ammo, such as Eley Tenex, shows much tighter tolerances. With no moving parts (other than the thumb screw), this simple gauge is easy-to-use and very repeatable. It also has a convenient lanyard you can loop around your wrist. You can learn more about this tool in a full RifleMagazine.com review.

Gereg’s second tool is a very nicely-crafted gauge that measures rimfire rounds from base of rim to the bullet ogive. The G3 MK II Pro tool fits comfortably in the hand while inserting a cartridge in the measuring chamber at the bottom of the tool. This G3 MK II Pro tool features a built-in dial indicator making read-outs quick and easy. Just slide a rimfire cartridge in the base of the unit and gently yet firmly push the round into the measuring chamber until the dial indicator comes to rest. The dial indicator on the gauge gives you a number which you can use to compare base to ogive lengths. Note: The piston is indexed to assure its return to the original setting. When we used the gauge with inexpensive ammo, we saw variances in rim base-to-ogive lengths of up to .025″. The high-end ammo, such as Eley Tenex, is much more consistent, with 80% of rounds falling within .008″ rim base-to-ogive length spread.

To purchase either tool, email Gerry Gereg, gerry.gereg@snet.net, or call 860-354-7500.

Texas Testing: Sorted vs. Unsorted Rimfire Ammo
Texas rimfire shooter Ben Peal ran a series of tests to determine how rimfire ammo sorted with the G3 tools performs vs. “out of the box”, unsorted ammo. Ben shot a series of 5-shot groups at 50 yards, after sorting the ammo for rim thickness and then rim base-to-ogive lengths. Ben tested four (4) types of ammo: Eley Target, Federal Champion, Remington Target, and SK Standard Plus. Ben’s tests, conducted with a CZ 452 rifle shot from the bench, produced some interesting results. Ben’s tests showed that sorting resulted in a meaningful reduction in average group size for all ammo types. In each case, the sorted ammo shot smaller than unsorted ammo (biggest improvement was with the SK Standard). Admittedly these tests are far from definitive because only one factory rifle was used. We certainly don’t claim that sorted ammo will shoot better in every rimfire rifle — and the benefits of sorting high-end ammo may be hard to quantify. But Ben’s tests do suggest that sorting may be worthwhile with low- and mid-priced rimfire ammo. Here is a chart showing Ben’s results:

Ben writes: “Using the ogive gauge, cartridges may be sorted into those of similar measurements, or a narrow range of measurements, allowing us to select cartridges to be chambered with the ogive at or near the same distance from the lands. Cartridges were to be grouped into two categories for test firing. Group one of each brand were cartridges ‘as received’ with no sorting, measuring, or weighing. This represents an accuracy baseline for each particular brand. Group two were cartridges first sorted by rim thickness and then separated into sub-groups based on rim base-to-ogive measurements. I included rim thickness to eliminate a controllable variable that could possibly affect accuracy. I wanted the groups from test firing to reflect the influence of rim base to ogive measurements only.”

I found a considerable difference between brands as I measured and sorted cartridges. Eley Target and Remington Target had the least variance in rim base-to-ogive measurements and very little difference in rim thickness. Federal Champion and Standard Plus had the largest percentage of cartridges longer or shorter than the norm and a wider range of rim thickness. Standard Plus turned in the largest average groups before sorting, Eley turned in the smallest. These two brands had near equal small groups after rim to ogive sorting.

The groups below were shot by tool-maker Gerry Gereg with his Winchester Model 52 and a 20-power scope. In that rifle the rounds with shorter rim base-to-ogive lengths shot best, as you can see:

You can learn more about the G3 rimfire gauges and Ben Peal’s testing by reading Ben’s full, 2940-word report. CLICK HERE to read Ben Peal TEST REPORT (PDF file).

Editor’s Note: Along with rim thickness, and rim base-to-ogive length, we have found that rimfire cartridge run-out (measured on the bullet) can have a dramatic effect on accuracy. In fact, in some of our barrels, we have noted that rounds with very high run-out (poor concentricity) often result in “flyers” that ruin a group. For this reason, we believe that concentricity checking is another procedure that can potentially benefit rimfire shooters.