RE-LOADING BASICS - Importance of seating primers correctly

With no end to the current shortage of primers in the UK in sight, we are all having to shop around and use whatever brands we can find. Changing the brand of primers that you use can cause a few issues, like seating problems and light strikes, and you may have to make some changes to your reloading process to overcome them. It is well worth remembering that switching to a different brand requires you to carefully consider the importance of primer seating depth.
There are a lot of different priming tools, including press-mounted, handheld, and bench-mounted options, and they all do the same job, but the preciseness of the primer seating depth is critical for producing top-quality ammunition.

Sizes and tolerances
Both primers and the pockets they sit in are made to extremely tight tolerances, but there are still very slight differences between the various brands, which can have a big impact on how well the primers seat. When it comes to these size differences, the diameter and depth of the primers and the pockets are important. If you have primer pockets that are small in diameter, although within tolerance, and primers that are large in diameter but also within tolerance, then seating them is going to be hard, and damaged primers can often be the result.
If the primer pockets are deep and the primers are on the shallow end of the permitted tolerance, the primers may sit too deep, meaning the firing pin does not reach or hit them correctly. They can also fail to sit correctly in the bottom of the pocket, so the feet of the anvil do not engage properly, which can also lead to light strikes. As a primer is seated, the feet of the anvil are pushed into the cup, forcing it up against the primer compound and ‘arming’ the primer. If this does not happen, then part of the force that the firing pin hits the primer with is used to fully seat the primer and there might not be enough energy left to actually detonate it.

Correct depth
What exactly constitutes the correct seating depth is much debated, and it is of course affected by the tolerances already mentioned, but somewhere between flush with the face of the case head and 0.008” below flush seems to be the generally accepted rule. Most rifles will have sufficient firing pin protrusion to reach and indent them enough to crush the priming compound so that it ignites. Sometimes, light strikes can be due to the firing pin protrusion being too short or the power of the pin being insufficient, rather than the primer seating being at fault.
If you cannot achieve the correct primer seating depth, you may need to change the brand of cases or primers that you are using. Alternatively, you could try a primer pocket uniformer or a cleaning tool to adjust the primer pocket dimensions. The slightest of changes to the primer pocket can often make all the difference.
Achieving the correct seating depth, provided that your particular combination of primers and brass is compatible, is relatively easy with any of the priming tools available. You really need to ‘feel’ the primer engage with the bottom of the pocket, and this is far easier with a handheld priming tool. With press or bench-mounted seating tools, it is harder to sense the primer hit home and seat properly. If your primer is still protruding, then you will need to apply further pressure to squeeze the primer down.

Doing the sums
You can actually calculate the correct seating depth for your primers by taking some measurements and working out the optimum theoretical dimension. You need to measure the primer pocket depth, the overall thickness of the primer, and the amount that the anvil protrudes.
If your primer pocket is 0.130” deep and your overall primer thickness is 0.125”, then you would have to seat your primers 0.005” below flush to achieve anvil contact with the bottom of the pocket. If the anvil feet protrude 0.010” from the cup, then you need to seat to 0.015” below flush so that the cup will bottom out in the primer pocket and the anvil will be pushed in and compress the primer compound. In this situation if you seat deeper than 0.015” below flush, then you are over-compressing the primer and could damage it.

Theory into practice
Having calculated the theoretical seating depth, putting it into practice can be tricky for two reasons. Firstly, the tools designed to specifically measure primer seating depth are very expensive. Secondly, your average priming tool does not have the facility to ‘set’ the depth or monitor the dimension. The seating depth is entirely dependent on the pressure that you apply each time.
The best approach is to calculate the correct depth, be it flush or below flush for your case and primer combination, and then seat a few primers and examine how they actually sit. If the primers should be below flush but are actually sitting flush with the face of the case head, try to reseat them applying slightly more pressure. This should make the primers sit at the correct depth, as calculated, but do not apply so much pressure that they crush and flatten. Likewise, if the primers are seating deeper than the calculated dimension, you should try applying less pressure, while ensuring you still feel the primer bottom out in the pocket.

Conclusion
Who knows when the current primer crisis will end, but there are still a few brands available here and there. If you check the dimensions of an alternative brand against your usual primer and adjust your seating depth accordingly, they should still function well and keep you shooting.