GROUP THERAPY: Have you ever had a flyer?
It would have been an amazing group except for that one shot. The first shots made a single ragged hole. But THAT shot was off just enough to ruin the group. Did you flinch, jerk the trigger, or time your breathing wrong? Who knows, but in the end you just missed making that perfectly tiny group. Was it your fault, or was there something else that was a factor? Could it have been the ammunition? YES! But, on that group, you'll never know. Hopefully this didn't occur during the final round of a benchrest competition, or when you're at the range bragging to your friends about how accurate your new rifle is!
With all the money that is spent making our rifles more accurate, it makes sense to examine the ammunition we use as well.
From now on, make sure you own the tool that will help reduce the likelihood of having an ammo related "flyer".
According to Eley, the manufacturer of the highest grade rimfire ammunition available (used in the Olympics), the single most important factor in rimfire accuracy is: the thickness of the rim.
The dial indicator and dial caliper are not included with the rim gauge. The dial indicator I have shown in the photos is a Smith and Wesson, 0.0005 x 1/4" indicator. When I purchased it, it was model #SW1073, if no longer availble there are many similar types. The dial caliper is a common tool that can be found at many online stores. An ideal increment for each is 0.0005 or 0.001 inches. Complete setup and usage instructions are in the PDF above.
So why is the rim thickness so critical?
Inside the rim of a rimfire cartridge is a percussion sensitive compound, the primer. When the rim is hit by the firing pin, the primer literally explodes and ignites the propellant. The initial jump of the bullet, caused by the primer, as the powder begins to ignite, right as the bullet engages the rifling is a critical stage for accuracy. This stage is directly influenced by the amount of primer present.
The rim thickness varies even within the same box of ammunition. Manufacturing processes get the dimensions similar, but not exactly the same. The differences we are talking about here are only a few thousandths of an inch. A small difference in thickness has a proven effect on accuracy as determined by top benchrest rifle champions.
How much difference can there be?
Manufacturing methods have improved dramatically over the last twenty years, however there are still limitations to their ability to produce geometrically exact casings. A typical rim thickness can be 0.037" (approximately 1/32"). Within a typical box of ammunition, you will find rim thicknesses between 0.036" and 0.040". More expensive ammunition tends to be more consistent, however variations exist even in high end products. Without a measuring tool, you have no way of knowing what you have.
When you consider that the brass casing has a thickness of around 0.010", and that there is a top and bottom layer of this brass that the primer is sandwiched in-between, you are left with a thickness of primer of around 0.017" for a 0.037" rim. If the rim is 0.041", the primer thickness becomes 0.021, or 24% greater than in a 0.037" rim. This variation in the amount of primer affects accuracy.
So, what is the ideal rim thickness?
Excellent question. The answer sounds basic, but the ideal rim thickness is a consistent rim thickness. You can obtain excellent groups with a rim thickness of 0.038" or with a thickness of 0.040", but you will not have excellent groups with a batch of shells that have both rim thicknesses. The key is consistency, and sorting is the only way to achieve this.