.22 LR Rim Thickness, homemade gauge and sorting results so far.

[thomae]

Sorting .22LR ammo by rim thickness has been mentioned, but I thought I would post how to manufacture a (partially) home made rim thickness gauge and talk about my results so far. I thought this might be interesting to those of you considering sorting your ammo by rim thickness.

I had about 700 rounds of Eley Sport (Inexpensive, but shot the best in my Savage MKII Heavy Barrel) that I wanted to sort.

I didn’t have a rim thickness gauge, so I decided to make one. Turns out it’s easy (if you have the right tools).

I already have a Hornady bullet comparator, so I have the red thingamagig (I’ll call it a “caliper base” since I don’t know the right word for it.) that attaches to the blade of my calipers.

I have a mini-lathe, so I chucked a piece of used .243 brass in the jaws and faced off both ends so they would be square.
(I immediately labeled it so I don’t get stupid and try to reload it some day.)

Here is a picture of the parts:



I put it into the Hornady caliper base (It fits quite nicely), and tightened the set screw to hold it in.
To measure rim thickness, zero the calipers on the empty .243 brass, than , and drop the .22LR round into the bullet-end of the .243 brass and measure the overall length with the calipers.

Here's what it looks like measuring:


One thing I noticed was that sometimes I needed to take two measurements to get a better idea of the average rim thickness. This is not difficult, I would measure, twist the .22 round about a quarter turn, and then measure again. Some rounds measured consistently regardless of how many times I turned and remeasured. Others varied depending upon where on the rim the caliper was.

Since my calipers measure only to the nearest .0005 inches, I sorted into the following categories:
.0360 - .0365” (about 5.7%: slightly less than 50 rounds)
.0370 - .0375” (about 43% - approximately 300)
.0380 - .0385” (about 36% - approx 250 rounds)
.0390” - greater (about 6.3% - slightly less than 50 rounds)

Note: I know the percentages don’t add up, but it wasn’t exactly 700 rounds…that’s simply the easy round number I used for my calculations.

FWIW: It would have been a closer count between the .037s and the .038s, but I dropped about 30 or 40 of the 038s on the floor
(Note to self: Try not to be a slapstick comedian: check both ends of your boxes and make sure they are closed before you pick them up!)

I have not yet had an opportunity to shoot my sorted ammo, so I have yet to see if the rifle actually prefers one rim thickness, or if the sorting simply improves consistency.

[Brick]

Interesting. Is this a way of optimizing headspace? Or some other dimension?
And, is there a standard that you've settled on as desirable? Or perhaps a rim thickness that performs best in your particular rifle? Or are you going to shoot test groups at a significant range (say 50 yards) to determine which is best? If the last I hope you post the results!

[thomae]

From doing a lot of reading, since .22 rifles headspace on the rim, consistent rim thickness equals consistent headspace which takes one variable out of the "why are my groups this big" question. Headspace gauges are used commonly by .22 benchrest shooters.

It would seem to me that there might be one particular rim thickness that performs optimally in any particular rifle or pistol.

or, it might be that by selecting ammo with more consistent rim thickness, you eliminate the variability. (What I mean is that hypothetically, perhaps you get the same size groups from two different rim thicknesses, but they are at different spots on the target)

I have yet to really do enough shooting to figure this out. When I do, I will post my results.

If there are any experienced .22 BR shooters who have tested the effects of variable rim thickness, please chime in...I don't pretend to be an expert on this and would welcome any additional corporate knowledge floating around.

In the mean time, be of good cheer