Experiment Time

Well, it snowed this weekend, so while cooped up in the house I decided to give the old rim thickness, cartridge weighing thing a try.  I know, I know its been done, done a few times over and the results are always the same: measuring rim thickness has no bearing on accuracy.  Ok, I believe all the results I’ve read so why bother?  Basically, I was bored and curious just for myself.  So I began with a single box of Federal Champion (525 rounds) bulk box ammunition, advertised as 36 grn copper washed hollow point bullets traveling 1260 fps.  First I decided to separate the ammo by rim thickness.  I utilized a dial indicator and a Raven Eye custom rim thickness gauge tool for this part of the process.   Here’s my setup:


The snow outside.

The snow outside.


Getting everything together.

Getting everything together.


I drew 50 rds from the box at random to use as a control.  I then measured the rim thickness of the remaining 475 rounds.  The sample broke down into 3 separate groups.

Group 1: This group measured between .34 and .35 and totaled 66 cartridges (13.9% of the sample size)

Group 2: This group measured between .35 and .36 and comprised the bulk of the sample size at 357 total cartridges (75.2% of the sample size)

Group 3: This group measured between .36 and .37 and contained 52 cartridges (10.9% of the sample size)

* I found it interesting that out of that lot of ammo there was an average rim thickness of 0.355 and  spread of 0.03.  Not too much it seems especially for bulk box ammunition.

I then set aside the cartridges from groups 1 and 3 and focused the second part of the experiment solely on group 2.  This group I proceeded to weigh and segregated the cartridges based on weight in grains.  The scale I used was a Franklin arsenal pocket scale. Now I realize this device is not a precision piece of equipment but it is what I had available for use.  I found the little boxes I bought from RFC sponsor Smartreloader.com to be very handy in this case.  The 357 cartridges broke down as follows:

48.9 grns: 4 rounds (1%)   49.0 grns: 4 rounds (1%)   49.1 grns: 26 rounds (7.3%)   49.2 grns: 50 rounds (14%)   49.3 grns: 178 rounds (49.9%)   49.4 grns: 50 rounds (14%)   49.5 grns: 38 rounds (10.6%)   49.6 grns: 7 rounds. (2%)

Average weight for the sample: 49.25 grns     Sample Spread: 0.7 grns

Ready for the range.

Ready for the range.

Finally, as an anecdote I chose a 50 rd box of the best quality match ammunition I had on hand, some old Federal Match UM1 to run a small comparison.  Now, I realize the comparison is not really fair as the sample size is much smaller but as I said it’s just anecdotal.  In short, of the 50 rounds measured the rim thickness of every round was in between 0.38 and 0.39.  The weight was 5 rds. were 51.7 grns, 14 rds. were 51.8 grns, 21 rds. were 51.9 grns, and 10 rds. were 52.0 grns.  for an average of 51.85 grns and a spread of only 0.3 grns.

Now, again while I recognize the sample size is much smaller we get a small glimpse at the quality difference between and the consistency of match ammunition vs. bulk.

Finally, I must note that I am drawing no conclusions to the accuracy capability as a result of my experiment nor am I making any broad sweeping statements.  I just performed the tests and leave the data for you guys to see and evaluate for yourselves.

(Stay tuned, yes, I am going to shoot this stuff and will post results when I do.)


Categories: Rimfire Roundup! | Leave a comment

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