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Raven Eye Custom

we are rimfire - PERFECTED

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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 hereare 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 thicknessof 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.

 

This gauge has been designed to measure the rim on 22LR and 17 Mach 2 ammunition, and will not work with 22 Mag (22WMR) or

17HMR. There is another gauge designed to specifically measure these rounds. See my other listings for details.

 

( 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.  The one with the lifting lever is an ancient Mituoyo that I have no idea the model number of.  However, many other models work well. Complete instructions are included in the gauge )

 

 

 

 

22LR Rim Thickness GaugeMeasures 22LR, 22, 22 Short and 17 Mach 2

 

Used with a dial caliper

 

 

Used with a dial indicator

 

 

 

 

CURRENTLY BACKORDERED UNTIL AUGUST 2014

 

 

 

 

 

 

  Usage instructions:

 

   Page 1   Page 2   Page 3   Page 4

 

22 Magnum Rim Thickness Gauge

Measures 22 Magnum and 17 HMR

 


 

 

Currently out of stock on the 22Magnum gauges. 

 


 

 

 

 

 

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