Does Primer Seating Depth Matter?

Updated: Jul 27

For the longest time, seating depth on primers and especially primer pocket uniforming, has been discredit by almost all of the shooting community. Unfortunately, this tedious method has been pushed off by many individuals.

Well, I decided to give it a shot despite what everyone has been saying in forums for many years.

For the nature of the test, I used a 6.5 Creedmoor with a Criterion Remage, 1/8 twist, 26" barrel, using Berger Hybrid 140's, Lapua brass, CCI 450 primers, and Shooters World Long Rifle. Full length L.E. Wilson Bushing die, CPS Primer seater from Primal Rights, 21st century primer pocket uniformer, 2 thousandth neck tension, and annealed every-time with the induction annealer from AMP. Side note, if you guys haven't checked out AMP annealers, you should. It has significantly lowered my SD and ES by annealing every firing as they recommend. Along with water volumetric weighing each case and uniforming the flash holes.

But, before we get started, let me explain what we are actually testing here. I first measure each primer pocket of my brass and uniform them all to the same depth. In this case, I uniformed them to -0.1215". I then took the primers themselves and measured those to be exactly 0.119" from the bottom of the cup to the top of the anvil itself by using the Accuracy one primer depth gauge. From here, after establishing a very good node and tuning things by altering the seating depth, I then proceeded to alter the "depth" of which the primers were seated.

How Does Primer Seating Depth effect Accuracy?

Starting off, I performed a ladder test, or OCW, in which I found 38.3 grains to yield a very good node. My average velocity was 2667 FPS, with an SD of 4 and an ES of 8. While I also seated all of my primers at -0.002" below flush. My picture below of my grouping at 100 yards, which yielded 0.292 MOA. As you can see, which is important to note, the one "flyer" which we will address in a few moments.

Now, after establishing my OCW with my "base" of an SD of 4 and an ES of 8, going 2667 FPS on average, I decided to alter my seating depth next to "close things up." Now, this is the part where most people would call it quits as I have for so many years. Moreover, because it has been taught for so long that these are the only two basic tests that one should do.

For my seating-depth test, I had produced an even smaller grouping with a slightly better SD and ES. I settled at 2.235" CBTO, with an SD of 2 and an ES of 4, going 2670 on average of three shots.

Does Primer Seating Depth Affect Accuracy?

This is where the magic comes in. This might be something that you seriously want to consider trying with your own load development, especially, with the affordability and variety of tools coming out that will allow you to adjust seating depth for primers. Even though this concept is not new, because some bench-rest shooters perform this step, I haven’t found any data out there that supports the concept of primer seating depth, itself. On another note, I will say that I have not tested anything other than the Primal Rights CPS, so I can't guarantee that you will receive accurate results to the one-thousandth like I am when seating primers. Even further, the same goes for primer pocket uniformers. If your primer pocket depths vary, then you will see inconsistent results before seating your primers. If you want to fix that, I'd encourage you to look into the tool I am using because it allows me to adjust the actual depth and is not preset. Moreover, for the seating depth test, I loaded 3 shot strings at 2.235" CBTO, 38.3 grains like I have been using originally, and only altering the seating depth of the primers themselves. In order: -0.0000, -0.0005, -0.001, -0.0015, -0.002 below flush. Now, let's dive into what I found that is groundbreaking to me. 

  • -0.0000 SD of 6, ES of 12

  • -0.0005 SD of 2, ES of 5

  • -0.001 SD of 1, ES of 1

  • -0.0015 SD of 8 and ES of 15

  • -0.002 SD of 4 and ES of 8.

So, what is the big deal about this? This shows that there is a sweet spot that you can find when altering your primer seating depth. Here, to which absolutely amazed me, was an SD of 1 and an ES of 1 at -0.001" under flush with an average speed of 2667 FPS.

Also, what I found, is that it literally eliminated the "flyer" in my group. Moreover, to my disbelief, I created a 0.048" with a three-shot group.

In conclusion, I know some people will discredit my findings because it has been pushed off for so many years by reloaders as something that does not matter. However, I really encourage those who have the time, to at least try it. Seriously, you have nothing to lose and everything to gain, here. Maybe this is a fluke, or maybe this is something legitimate that more people should be paying attention too. With a finding like this, if you do test it, make sure that you have the most "consistent" hand-loads as possible.

In my testing I measured to the granule of powder with the RCBS Matchmaster, and used some pretty sophisticated equipment too, like the AMP, Primal Rights CPS, L.E. Wilson Hand dies with the Arbor Press that produces, for me, 0.001" runout or less, and the Accuracy One Primer Depth Gauge. Testing with "cheaper" components, did not yield results as good as these. So, results will vary on your ability to produce "good" ammo to begin with and having your marksmanship fundamentals down. I would also encourage you to report your results back to me. While I found this to work for me and confirmed my groupings, SD, and ES on three separate days from ranging temperatures of 73-90 degrees Fahrenheit, I can't say that statistically, this is something that is groundbreaking for the shooting community until more people, with an unbiased opinion, that does not get paid to alter findings, report their conclusions back in to make an analysis of the results from across the board.

However, does primer seating depth matter and affect accuracy? Yes, it did for me at-least.

On another note, can you make ammo that is amazing without altering the seating depth of primers? You bet, no doubt about it. People have been doing it for years. However, I personally am always looking to improve my hand-loading processes more each day. I can see this step being utilized in a bench-rest scenario because now, we are dabbling on the concept of hitting targets vs shooting the tiniest hole possible. Which, this can be argued from a million different perspectives: Is it worth it for certain ranges? Is it worth it for competing in certain divisions?, etc. However, I think everyone could benefit by closing their groupings up and shooting better SD's and ES's. Especially, if they prepare their hand-loads with the utmost precision as possible with some of the best tools around that allow the individual hand-loader to do so.  Side note: I do not mean to discredit the ability, by any means, to make good ammo with "lower-priced point components and tools."  This is why when I explain hand-loads, I put the terms good, consistent, cheaper, all in quotation marks because ultimately it is completely subjective. However, for a test like this to see the proof of the results, it took a lot of work and severe OCD.

Until then, I will be messing with primer seating depth more often in my load development because I personally received some amazing results.

All in all, give it a shot and see if it works for you.

As always, shoot straight, be safe, and happy reloading! If you liked this article and think it was worth $1, please consider donating:

Blake has been writing reloading articles for three years and helping out within the community to further enhance reloading education. In his free time, he works within the community to help out new hand-loaders by educating them on the many variables that come with this wonderful hobby. His passion is solely based on helping others so that they may pass on that information to future generations, keeping the art of hand-loading alive.         

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