Benchrest accurizing products
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.
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, before the rifling is engaged 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 sandwitched 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.
22LR Rim Thickness Gauge
Measures 22LR, 22, 22 Short and 17 Mach 2
$14.99 $13.69
plus $3.15 shipping
22 Magnum Rim Thickness Gauge
Measures 22 Magnum and 17 HMR
$14.99 $13.69
plus $3.15 shipping