Primers: Seating Pressure and Pre-Compression

I recently received the following note from a friend overseas.  Since the question and answer have broad applicability, I'll put them here for future reference.  I'll try to take and post some pictures for this in the next week or so, but it wasn't really something I had planned and I'll have to scrounge up a camera with macro capability from a friend so it might take a while.

Hello Germán ,
I have enjoyed reading a few of your posts and articles about primers among other things. I read a post recently were someone has been seating their primers way too hard and after some measurements, it seems my big clumsy fists have been squeezing the primers a bit too hard as well, giving about .008" crush at least. On the rare occassion I have been able to test over a chrono, I had higher than expected ES, so I am thinking it could have been the primers jammed in way too hard?

I mainly use the Russian SRM and LR primers in my 6x47 Lapua and my .284. What sort of crush should we be looking for with these primers?

Thanks for any help, from Rod D.
Hello Rod,
Primers have a definite "sweet spot" in terms of seating pressure and pre-compression of the pellet. These days the manufacturers give out essentially no technical information on the properties of their primers, we're all taken to be semi-useful fools, I suppose. However, at one time, Federal used to recommend a "light pre-compression" now that's a pretty vague statement, but better than silence.

Creighton Audette tried to get some numbers from Federal and got nothing more from them. He then designed what is now sold as the K&M seating tool that allows you to measure how much pre-compression (crush) you are applying to the primer (K&M also makes a version without the gauge). Of course, the system depends on all primer pockets being uniformed for depth. After a great deal of experimentation with pre-compression and related accuracy testing, Creighton determined that there was no specific number, but you could definitely do too much or too little. Very ironic, a light pre-compression indeed turned out to be best... whatever that means. No one was better at this sort of testing than Creighton, by the way. I'm as chagrined at the non-numberical result as he was but not knocking his work in any way.
I use the Sinclair tool which is another version of the Audette design, although without the crush measuring ability. Both the Sinclair and the K&M give the best feel for what's happening as the primer seats of all the tools I've tried.

In my own testing, I have found that the best method is to clean the primer pocket (I don't uniform them) then, seat the primer until you feel it just touch, then apply a bit more pressure (not a lot) until you feel a second level of resistance just starting.

Photo from the Remington website shows how the anvil will pre-compress the pellet on seating.

If you examine some new primers you'll see that the legs of the anvil stick out past the bottom of the cup. When the legs hit the bottom of the primer pocket you feel that first resistance. As you continue to apply pressure you bring the cup over the legs and then the second level of resistance begins as the center of the anvil begins to compress the pellet. Stop!

The primer pellet is very small, it typically weighs 0.5 gr. for a large rifle primer and it can be cracked through the application of too much seating pressure. However, it needs a tiny bit of pressure as the tip of the anvil should be in contact with it when the firing pin hits - it shouldn't have to move at the firing pin's impact to make contact with the anvil. If it has to move, or if it has been cracked, SD will increase.

To continue a bit with the unfortunately non-numerical approach, if the primer loses its edge radius on seating and looks somewhat concave, that's way too much pressure. A bit of flattening is normal and correct, but like Goldilocks testing the bears' mattresses, there's too soft, too firm and just right.

A worthwhile test might be to try chronographing three sets of loads, one in which you stop as soon as you get initial resistance (legs at the pocket bottom) one where you crush mightily, and one where you add a slight pre-compression as described above. The Goldilocks test so to speak. I would be very interested to hear your results and will do the same test myself next time I chronograph something.


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6852 Lakeshore Road
West Olive, MI 49460
Telephone: 616-399-7894

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