Primer pocket

The ever-growing problem with primer pockets

To ensure correct ignition, primer pockets need to be of the correct depth, clean and of the correct size. After a case has been fired, it is obvious that the pocket needs some attention to bring it back into acceptable condition.

With normal use, cartridge cases will grow lengthwise, and at the other end of the case, the primer pocket will grow laterally. Quite often, certain tools will not enter a new cartridge case primer pocket. 

Small and large rifle primer pocket uniformers

The rate of growth depends on the quality of the brass, its ductile properties and the strength of the loads used.

Before reloaders annealed brass cases, it was normal to get somewhere between five and seven reloads before the ductile properties of the case neck were so low that it would not hold a projectile. 

Some cases never made it that far and split the necks at the third or fourth cycle. 

Annealing prevented both problems but created another, which is not much talked about. It appears primer pockets continue to expand laterally, very slowly, for the life of the case. 

Eventually, the pocket will not hold a primer and the case will have to be scrapped. We will return to this later.

You cannot resize the primer pocket but you can clean it to ensure proper combustion when reloaded. 

Once the fired case is de-primed, I use a tool called a primer pocket uniformer. The whole idea of this tool is to cut primer pockets to a uniform depth and square to the case head. The head of the case provides the datum, so you are unable to cut too deep with the normal, non-adjustable tool (some tools are adjustable).

Priming with a hand tool is a sensitive way of ascertaining pocket expansion by ‘feel’

I attempt to use this tool with new cases to ensure that the pockets are square, but often the pockets are so tight that the tool will not fit. To some extent, this depends on the quality of the brass. 

The tool also performs another useful function: it cuts the black combustion residue in the pocket. It may leave a small amount of hard material in the bottom of the pocket, which you can get rid of with a wire brush. 

In my experience the rounded internal edges of the pocket may result in the removal of a very small amount of brass with each use.

Another method of cleaning the pocket is with a specific-sized wire brush. Of course, further cleaning takes place when the case is subjected to immersion in the ultrasonic cleaning solution; every little bit helps.

With annealing extending case life into unknown figures, it occurred to me that some method of measuring primer-pocket size was necessary. 

The Swage Gage at the bottom the photograph with the ‘go’ end in the pocket, indicating correct size. The black circular mark to the left of the case head will show correct depth

One is by using a hand tool to prime cases, so you can easily note the lack of resistance when seating a primer in an individual case.

I hunted around the internet and found a company called Ballistic Tools makes all sorts of measuring gauges for brass cartridge cases. The Swage Gage large rifle primer-pocket gauge is simplicity itself. One end is a go-gauge, and the other is a no-go gauge. The go-gauge indicates the correct primer pocket depth; if the no-go gauge fits in the pocket, the case has to be scrapped. They can be found at

Finally, from time to time, you hear of off-centre flash holes. I suggest it would be an interesting experiment to see the effect on both velocities and groups that it might produce.




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Ron James

One Comment

  1. Hi there, good article and thanks for the shout out about the swage gage. I think it would be interesting research to see what effect off center flash holes has. I know bench rest shooters will put a lot of effort into flash hole uniforming sometimes. – Gigs @ Ballistic Tools