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Primer Part

In part one of this series on primers, we looked at that the different primers are and what the primer actually is. In part two, we will be getting more in depth and looking at how the primer selection can affect your reloading success.

How a primer affects your load

Primers affect the pressure generated by the cartridge. Changing from standard to magnum primers may substantially raise the maximum average pressure of the cartridge and indiscriminate changes are not recommended. The A-Square Company conducted pressure tests involving six different primers. These tests used the 7mm Remington Magnum cartridge with a 160 grain Sierra BT bullet and 66.0 grains of H4831 powder and the results were reported in the A-Square reloading manual Any Shot You Want. A-Square used CCI 200 and 250, Federal 215, Remington 9 1/2M, and Winchester WLRM and WLR primers in these tests. They revealed a total spread in pressure of 12,800 psi from the mildest standard (the CCI 200) to the hottest magnum (WLRM) primer tested.

Changing brands, but using the same type of primer will also usually result in pressure changes, but ordinarily these will be less drastic. In the A-Square tests the pressure spread between the CCI 200 and the hottest standard primer (the WLR) was 9600 psi. The spread between the mildest magnum primer (the Rem. 9 1/2M) and the hottest magnum primer (WLRM) was 8300 psi. These are significant pressure variations that cannot safely be disregarded.

Incidentally, these same tests revealed that the Federal 215 and CCI 250 large rifle magnum primers produced nearly identical pressures. The difference between these two primers was only 100 psi. A-Square also reported that, while they had not tested these two primers in all possible cartridges, this result was typical of their experience with these two primers.

It’s a good idea to follow the recommendations of whatever reloading manual you are using as a reference regarding the proper brand and type of primer to use. For example, if the load in the manual was developed using a WLR large rifle primer, then that is what I use. If one brand of primer (of the same type) must be substituted for another, the conventional wisdom is to reduce the recommended powder charge by 10% (assuming the load is not already at the minimum) and work back up slowly.

Seating the primers

When reloading, always seat primers slightly below flush with the head of the cartridge case. This insures that the anvil is properly pressed against the priming compound for reliable ignition. Failure to properly seat primers is the biggest single cause of misfires in reloaded ammunition. A good depth to aim .005" below flush. With some experience this can be determined by feeling the case head after the primer is seated. Any primer that is flush or protruding should be very carefully removed and the case reprimed. Decapping a live primer can set the thing off, so behave accordingly and take all necessary precautions, including ear and eye protection.

For hunting and general recreational shooting purposes I have not ordinarily seen a significant difference in accuracy attributable to using different brands of primers. However, there are exceptions to this general observation. Some loads seem to work better with a certain primer. Sometimes the chronographed standard deviation in the velocity of a load is lower with one brand of primer than with another. In such cases I just go with the flow and use whatever primer works best.

Modern primers are a marvel of ingenuity and production uniformity. The importance of these little cartridge "spark plugs" is often overlooked by recreational shooters--in itself a tribute to their reliability.