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Smallbore Rim-Thickness Gauges: We Like Bald Eagle

We test three devices to see which one works the best and is easiest to use. We also determine if the effort is worth it.

We wanted to find out for ourselves if rim-
sorting improved the accuracy of a variety of
ammo when shot from a top benchrest gun.
Shown are the $85 Bald Eagle rimfire gauge,
which we recommend, Sinclair International
sorting block, and RWS R50 ammo.

Many .22 LR bench-rest shooters swear that a technique they employ makes their performance on the range improve: They sort their rounds by rim thickness. Supposedly, sorting ammo by rim thickness makes it shoot more consistently and eliminates flyers. The reasoning is as follows: Since the headspace for rimmed cartridges is determined by the thickness of the cartridge case rim, variations in .22 ammo rim thickness can cause subsequent variations in accuracy. If the headspace changes shot to shot because of rim-thickness differences, a gun will fire .22 rounds inconsistently.

In theory, this approach sounds good, but it also seems too easy. If rim thickness is the magic dimension of .22 accuracy, then, it seems, we should be able to sort inexpensive ammo (which typically has a great deal of rim-dimension variance) and make it shoot dots. Or, we should be able to take lot-tested, accurate ammo in a given gun and make it shoot even better by sorting it by rim thickness. Having had many reader inquiries about these questions, we decided to find out for ourselves if rim-sorting improved the accuracy of a variety of ammo when shot from a top benchrest gun.

How We Tested
We used a benchrest-quality bolt-action Cooper Model 36 BR-50 in this test. The $1,850 Cooper has a match chamber and a 1-in-16 twist free-floated, glass-bedded 22-inch stainless barrel. The 6.8-pound gun has a black synthetic benchrest stock with a square forend and no cheekpiece. Marketed for use in competition BR-50 matches, it comes with a Jewell two-stage adjustable trigger, a rubber Pachmayr recoil pad, a competition step crown, and twin extractors on the bolt face.

To test how certain rounds performed in the gun when rim-sorted, we used ten lots of ammo that exhibited different degrees of accuracy in the Cooper. (See the July 1996 issue for an accuracy evaluation of 33 rimfire brands). We sorted four boxes of each ammo (200 rounds) by rim thickness using three different gauges from Neil Jones, Bald Eagle, and Sinclair International. We then shot accuracy groups with each ammo lot, comparing the performance of sorted groups (those which had enough rounds of a given thickness—at least 30 rounds, or 15 percent) against unsorted groups we handpicked by dimension. Usually, we would pick the most common dimension in a lot and shoot four rounds, then use a round with a different thickness measurement for the fifth round. For instance, in the Remington sample, 20 of every 50 rounds had a rim thickness of 0.041 inch, and one round in 50 had a thickness of 0.038 inch. We would introduce the “flyer” round at random times during the five-shot string (the shooter didn’t know which round was which) to see if it would indeed open up the group size.

We shot the test using Pro-Shot Bench Rest Targets. We mounted the targets on a cardboard backer and placed them at the end of a 100-yard enclosed tube. This ensured that wind and changing light conditions wouldn’t affect the data. We bench-fired the gun off a Ransom Rest front benchrest and a rear bunny bag. Our shooting sequence was to clean the Cooper’s barrel, then fire fouling and sighting rounds as needed, usually about 15 rounds. Then we fired record groups. We kicked out and reshot groups where the test shooter felt he had broken a bad shot.

We chose ten lots of ammo to test in the Cooper, including Fiocchi Pistol Super Match Lot 0135020, which was the top-performing round in our July test. The ammo sells for $13.50 a hundred. Other rounds included Federal Gold Medal Match Lot 308 ($13/100); RWS Special Match Lot 467CM127 ($14/100); Fiocchi Rifle Super Match Lot 013509 ($13.50/100); CCI Green Tag Lot K10A09 ($8.25/100); Remington Target 22 Lot W20N1C ($3.50/100); Federal Gold Medal Target Lot 3BH354 ($5/100); RWS R50 Lot 4456R31 ($17.50/100); RWS FP50 Lot 445GK31 ($17.50/100); and Winchester T22 Target Lot 2KH01N ($3.75/100).

The Results
The Fiocchi Pistol Super Match Lot 0135020 sorted to four thicknesses. Sixteen percent (8 per 50 rounds) had rims 0.040 inches thick; 64 percent (32 per 50) were 0.041 inches thick; 18 percent (9 per 50) were 0.042 inches thick; and 2 percent (1 per 50) were 0.043 inches thick. Five-shot groups using ammo with rims 0.040 inches thick averaged 1.42 inches in size. Rims 0.041 inches thick shot groups 1.44 inches wide, on average. The Cooper performed best with rims 0.042 inches thick; those groups averaged 0.97 inches in diameter. Next, we tried to see if mixing bullets of different rim thickness made a noticeable difference downrange. When we mixed 0.041 rims and 0.042 rims, which accounted for 82 percent of the rim thicknesses, the group sizes averaged 1.21 inches. Similar mixtures of 0.041 with 0.040 and 0.043 ammo yielded groups between 1.15 and 1.50 inches—very close to the range of the sorted ammo. In our earlier test, the Pistol Super Match shot average groups of 0.67 inches. Thus, we saw little improvement rim sorting made in the Cooper’s ability to shoot Fiocchi Pistol Super Match Lot 0135020.

The Federal Gold Medal Match Lot 308 sorted to one thickness: 0.044 inches. Every round in this lot measured between 0.0440 and 0.0444 inches. (Just for fun, we also checked other Gold Medal Match lots we had on hand, 235 and 1304, and found they had the same dimensions and consistency. A third lot, 1615, had a 100 percent rim thickness of 0.043 inch). In our July test, Federal Gold Medal Match 308 shot average groups of 0.95 inches. Three groups we shot with the ammo this time were even better: 0.77 inches, on average. Because we didn’t have variations in rim thickness, we couldn’t conduct our flyer-round test. We concluded that rim sorting Federal’s Gold Medal Match 308 didn’t show us anything lot testing wouldn’t have found.

The RWS Special Match 467CM127 sorted to three thicknesses. Twenty-eight percent (14 per 50 rounds) had rims 0.040 inches thick; 54 percent (27 per 50) were 0.041 inches thick; and 18 percent (9 per 50) were 0.042 inches thick. Five-shot groups using ammo with rims 0.040 inches thick averaged 1.49 inches in size. Rims 0.041 inches thick shot groups 1.11 inches wide, on average. Rims 0.042 inches thick averaged 1.57 inches in diameter. Next, we mixed bullets of different rim thicknesses.

Combining four shots of 0.040-inch rims with one round of 0.042-inch rim thickness yielded average groups of 1.18 inches. However, the 0.042s weren’t the wide shots in any of the groups. When we mixed 0.041 and 0.042 rims, we got average group sizes of 1.35 inches. When we shot 0.041 and 0.040 rims together, the average group sizes measured 1.81 inches. In our earlier test, RWS Special Match 467CM127 shot average groups of 0.87 inches. Thus, we couldn’t assign any improvements in the RWS Special Match 467CM127’s accuracy to rim sorting.

The RWS R50 4456R31 sorted to two thicknesses. Two percent (1 per 50 rounds) had rims 0.040 inches thick; 98 percent (49 per 50) were 0.041 inches thick. Five-shot groups using ammo with rims 0.041 inches thick averaged 1.28 inches in size. In our earlier test, R50 shot average groups of 1.50 inches.

But when we mixed bullets of different rim thicknesses, we found a substantial performance upgrade. On one target, we combined four shots of 0.041-inch rims with one round of 0.040-inch rim thickness. The odd 0.040 round, first in the five-shot string, grouped with three 0.041 bullets in a 0.57-inch group. The fourth 0.041-inch round (fifth shot), opened the group size to 0.65 inches.

We viewed these results as mixed. Rim sorting enabled us to shave a quarter inch off our July results, but known mixed groups shot much better overall than any other groups we shot with the RWS R50 4456R31.

The Fiocchi Rifle Super Match 013509 sorted to five thicknesses. Two percent (1 per 50 rounds) had rims 0.039 inches thick; 10 percent (5 per 50) were 0.040 inches thick; 44 percent (22 per 50) were 0.041 inches thick; 36 percent (18 per 50) were 0.042 inches thick; and 8 percent (four per 50) were 0.043 inches thick. We shot sorted groups with the 0.041 and 0.042 ammo. Five-shot groups using ammo with rims 0.041 inches thick averaged 1.28 inches in size. The 0.042 groups averaged 0.86 inches in size.

When we mixed bullets of different rim thicknesses, we found little performance change. On one target, we combined four shots of 0.042-inch rims with one round of 0.040-inch rim thickness. The odd round, last in the five-shot string, grouped with three 0.042 bullets in a 0.66-inch group. The four 0.040-inch rounds had a slightly larger group size of 0.67 inches. On another target, we combined four shots of 0.040-inch rims with one round of 0.043-inch rim thickness. The 0.043 round, last in the five-shot string, grouped with three 0.040 bullets in a 0.83-inch group. The four 0.040 rounds grouped at 1.23 inches. In another trial, we combined four 0.042s with one 0.039 rim. The 0.039 round, shot third in the five-shot string, grouped with three 0.042 bullets in a 1.17-inch group. The 0.040 rounds grouped at 1.35 inches.

Likewise, when we shot four rounds of 0.041 with one 0.043 round, the “out” round was squarely in a four-shot set that measured 1.11 inches, while the four 0.041 rounds grouped at 1.68 inches. In our July test, Fiocchi Rifle Super Match 013509 shot average groups of 1.08 inches. We concluded that rim sorting didn’t improve the Fiocchi Rifle Super Match 013509’s accuracy.

The RWS FP50 445GK31 sorted to two thicknesses. Ten percent (5 per 50 rounds) had rims 0.040 inches thick; 90 percent (45 per 50) were 0.041 inches thick. Five-shot groups using 0.041 ammo averaged 0.76 inches in size, less than half the size in our July test, when RWS FP50 445GK31 shot average groups of 1.51 inches. Also, when we mixed bullets of different rim thicknesses, we saw slightly larger groups. Four shots of 0.041-inch rims combined with one 0.040-inch round yielded 0.96-inch groups, on average. The “out” shot was usually the 0.040 round. Sorting did improve the performance of RWS FP50 445GK31, we thought.

The Remington Target 22 W20N1C sorted to six thicknesses. Two percent (1 per 50 rounds) had rims 0.038 inches thick; 4 percent (2 per 50) were 0.039 inches thick; 26 percent (13 per 50) were 0.040 inches thick; 40 percent (20 per 50) were 0.041 inches thick; 26 percent (13 per 50) were 0.042 inches thick; and 2 percent (1 per 50) were 0.043 inches thick. We shot sorted groups with three thicknesses. Five-shot groups using ammo with rims 0.040 inches thick averaged 1.75 inches in size. The 0.041 groups averaged 1.97 inches in size, and the 0.042 groups measured 2.30 inches on average. When we mixed Remington Target 22 W20N1C bullets of different rim thicknesses, we found substantial performance changes. In one example, we combined four shots of 0.042-inch rims with one round of 0.038-inch rim thickness. The “out” round, first in the five-shot string, created a 2.63-inch group. The four 0.042-inch rounds had a slightly smaller group size of 2.45 inches. When we mixed 0.040 and 0.043 rims, the 0.040 shots grouped in a 2.22-inch pod, while the 0.043 shot ballooned the total group size to 3.10 inches. Obviously, sorting the Remington Target 22 W20N1C ammo helped its accuracy. In particular, the Cooper shot 0.040-inch rims the best.

The Federal Gold Medal Target 3BH354 sorted to two thicknesses. Twenty percent (10 per 50 rounds) had rims 0.042 inches thick; 80 percent (40 per 50) were 0.043 inches thick. Five-shot groups using ammo with rims 0.042 inches thick averaged 1.14 inches in size. The 0.043 groups averaged 1.34 inches in size. When we mixed Federal Gold Medal Target 3BH354 bullets of different rim thicknesses, we found little performance change. Four shots of 0.043-inch rims mixed with one round of 0.042-inch rim thickness created 1.26-inch groups, on average. Half the time, the 0.042-inch “out” rounds were the widest shots in the groups. In our earlier test, the Target ammo averaged 1.48 inch groups. Sorting the Federal Gold Medal Target 3BH354 seemed to improve its accuracy.

The CCI Green Tag K10A09 sorted to three thicknesses. Twenty percent (10 per 50 rounds) had rims 0.041 inches thick; 74 percent (37 per 50) were 0.042 inches thick, and 6 percent (3 per 50) were 0.043 inches thick. Five-shot groups we shot using ammo with rims 0.042 inches thick averaged 1.69 inches in size. When we mixed different rim thicknesses of CCI Green Tag K10A09 ammo, we found they outperformed the sorted ammo. Mixtures of four 0.041-inch rims mixed with one round of 0.043-inch rim thickness created 1.26-inch groups, on average. Mixing 0.042-inch rounds with 0.041 rounds produced 1.35-inch groups, on average. Mixing 0.041-inch rounds with 0.043 rounds allowed us to shoot 1.13-inch groups, on average. In our earlier test, the CCI ammo shot 1.20-inch groups. Sorting the CCI Green Tag K10A09 ammo didn’t improve its accuracy, we thought.

The Winchester T22 Target 2KH01N sorted to four thicknesses. Four percent (2 per 50 rounds) had rims 0.038 inches thick; 14 percent (7 per 50) were 0.039 inches thick; 50 percent (25 per 50) were 0.040 inches thick, and 32 percent (16 per 50) were 0.041 inches thick. Five-shot groups using ammo with rims 0.040 inches thick averaged 2 inches in size. Five-shot groups using ammo with rims 0.041 inches thick averaged 1.59 inches in size.

When we mixed different rim thicknesses of Winchester T22 Target 2KH01N ammo, we found they outperformed the sorted ammo. Mixtures of four 0.041-inch rims mixed with one round of 0.038-inch rim thickness created 1.43-inch groups, on average. Mixing 0.040-inch rounds with 0.038 rounds created 1.61-inch groups, on average. Mixing 0.041-inch rounds with 0.039 rounds allowed us to shoot 2.23-inch groups, on average. In our July test, the Winchester fodder 2.59-inch groups. Sorting the Winchester T22 Target 2KH01N ammo improved its accuracy, we thought.

The Final Tally
Overall, we don’t think it was worth the effort, time, and money we expended to sort our rims. However, the Winchester T22 Target, Federal Gold Medal Target, RWS R50, RWS FP50, and Remington Target 22 rounds did show improvement when sorted. The Remington ammunition showed the biggest gain. The other ammo, Fiocchi Pistol Super Match, Federal Gold Medal Match, RWS Special Match, Fiocchi Rifle Super Match, and CCI Green Tag, didn’t benefit from sorting, we thought.

However, the rounds that gained from sorting benefitted only in minor ways, we think. In some cases, the distribution of rounds was so thin in some samples that they couldn’t be used effectively in competition. For example, 18 percent of the Fiocchi Pistol ammo shot under MOA in our test. But that’s not enough of the sample to be useful in a practical sense, we think.

More realistically, sorting might be used to remove wild-shooting rim thicknesses. However, when we mixed rounds intentionally, more often than not what we thought might be “out” rounds were in the center of groups.

Because some competitors believe smallbore ammo sorting is effective—but our results were inconclusive at best—we asked other knowledgeable members of the shooting community what they thought of rim sorting.

Macky Locklin, a Corpus Christi, Texas, resident and top-shelf benchrest competitor, said he no longer sorts by rim thickness. The reason: “Several years ago, we didn’t have the quality of ammo we have now. To get good ammo required sorting by weight and rim thickness,” he said.

He said consumers put pressure on manufacturers for better ammunition, and six or seven years later, they have gotten it. “Manufacturers have gotten the ammo to the point that if there are still benefits from sorting, they are fairly negligible.” He also points out that weight consistency has pushed accuracy along more than rim thickness. “The ammo has gained from better weight categorization,” he said. “But I still sort by weight. If I’ve got something important coming up, I’ll sort my competition ammo on a digital scale. I used to be able to get 20 percent more accurate performance by sorting, but I don’t get that much anymore. I never got more than 4 to 6 percent accuracy gain from rim sorting, and that was after weight sorting. In the Eley Bench Rest ammo, variation in rim thickness has become so minor, that from a standard, I won’t see half a thousandths either way. A lot of people shoot theirs straight out of the box.”

Likewise, Dave Longren, Federal’s vice president for engineering, said his company looks long and hard at every possible aspect of bullet performance, and that rim sorting isn’t the holy grail of accuracy. “There are more than 500 factors that contribute to a product’s accuracy,” he said. “We strive to control as many of them as possible. We wish rim sorting was an effective way to produce more accurate .22 ammunition, because it is one of the production factors we can control very well. However, our tests have shown rim thickness is just one of many factors that make .22 ammunition shoot well. If you select the right ammunition, it’s not a consideration.”


Also With This Article
Click here to view "Rim-Sorting Devices."
Click here to view the accuracy tables.
Click here to view contacts and addresses.


-By Todd Woodard





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