At the 2015 National Matches Eley gave a presentation about the various stratagems used by shooters, such as weighing, or measuring rim thickness, to find the most accurate rim fire ammunition. All were bogus, whatever group size reductions announced by the prophets of these techniques, are founded on small sample sizes and
expectation bias Eley put up a chart of the physical characteristics they measure and control, and it was at least 80 dimensions shown, perhaps more.
No one at home is going to find the most rimfire accurate ammunition in their firearm by making correlations between weight, on a home scale, or taking random measurements, be they rim thickness, or some other arbitrary location. The only way to determine best accuracy is by shooting the stuff in your rifle, and shooting group sizes large enough to distinguish between one lot/brand and another. There is a case to be made for 40 shot groups, which incidentally, is the group size both Eley and Lapua use in lot testing ammunition for match shooters.
Take a look at the Oct 2014 Shooting Sports magazine
media/1533525/1014_ssusa_archive.pdf
The last article , page 38, is a reprint of a May 1958 American Rifleman article
"Assessing the Accuracy of 22 lr Target Rifles."
This article is based on test data, it is a very well planned and executed article. Smallbore prone shooters shoot 40 shots per "match", two targets of 20 shots for record each "match", and there are typically three to four "matches" to determine the aggregate. Smallbore prone targets have four record bulls for 50 yards, two record bulls for 100 yards. This is to help the scorer and the shooters.
You can understand for both the scorer and shooter, just how impossible it is to determine what is going on when five shots will cluster like this. For the scorer, are there any missing shots?, what shots are X's versus 10's when all you have is a ragged hole.
For the shooter, you need to spread the shots out so you can tell what is happening to the bullets as you shoot. So for small bore, each target is worth 200 points, each match is the aggregate of 40 shots, so for small bore prone shooters, they want perfect 40 shot groups.
Actually they want perfect 160 shot groups, and they want the groups to be less than the X ring in size. What you find, if you ever have your rifle tested at the Lapua test center in Mesa AZ or at Eley, is that lot selected ammunition will hold the ten ring at all distances but not the X ring.
What the 1958 American Rifleman article shows is that if a 40 shot group is the standard, then at 50 yards (the analysis goes out to 200 yards) than a 20 shot group is 88% the size of a 40 shot group, a ten shot group 71%, and a five shot group 57% of the size of a 40 shot group. Interestingly, at 100 yards, a prone with a sling group is 38% larger on average than if the group is shot bench rested. Small size groups tend to be smaller than large shot groups, it is the way statistics works out, and it takes lots of ammunition on paper before you can really trust your ammunition.
What you read in Gunmagazines, the typical three round, or the now "extreme Gold Standard" of five round groups, are totally worthless for actually trying to determine inherent accuracy. Gunwriters are paid a fixed fee, about $400 for each article, the less rounds they shoot, the more they get to keep, and they are not really interested in spending money determining the intrinsic accuracy of the article under test, they are interested in maximizing the amount of money they keep, and in seeing a sales rise for their customer, after the article. They are, after all, always looking for future work. But understand, what you read in print, is an old version of what you see on Cable Television Home Shopping Club. You are going to be taught
only what you need to know to buy. It is called advertising.
You need to shoot enough, and at distance, to have any real idea of how the ammunition performs in your weapon. You need to shoot enough to surface the flyers and dropped shots. A bud of mine, shooting F Class, got enough low velocity drops with TAC-22, that he stopped shooting the stuff.
He started shooting Eley Black box after that, and started shooting cleans.
Here are two targets, from a Regional I attended, one target is from the Match Winner, the other from a two time Smallbore National Champ. Both of these shooters have shot enough rounds through their rifles, that they have confidence in their ammunition, and they shoot groups like this, prone with a sling, with irons:
I have asked a number of very good shooters, just how many rounds it takes before they "trust" their ammunition, generally it is a brick of 500 rounds, and I will say, that is about right. Around 500 rounds downrange, in varying conditions, if the stuff is good, it will stay good. If the stuff throws rounds, or you experience catestrophic low velocity shots, such as that TAC-22, then, you will see it.
