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I Failed to Prove Seating Depth Matters

Updated: Dec 13, 2022

I failed to prove seating depth matters

Today I wanted to walk through a test I recently did in regard to seating depth. To preface things, a lot of times you will see individuals choosing a seating depth based on lower sample size testing and the thought process behind it is—that there is a vibrational node or timing that comes with seating a projectile at a certain depth. From there, a seating depth test is conducted where if you start jumping in .006” increments, or in other words seating the bullet deeper by .006” you will see your groups start to tighten in at some point. Moreover, while conducting a seating depth test, it is usually taught that if the first two aren't touching, then you go ahead and skip the next seating depth that was next to test, and shoot the next .006” deeper to see if the bullets touch. If they do, you continue to shoot. However, there are a lot of questions I had with this personally as it does not align with the topic of true dispersion of groups as seating depth tests usually aren't retested in great length after picking a pre-determined depth or even a "bad depths" with large sample sizes and can commonly get confused with being statistically significant.

What I mean by that is that even if a group appears to be bad with two shots, it doesn't mean that the group can't “close up” by firing more rounds at that specific seating depth at multiple targets. The same can be said with shooting a very good group. That group, if retested multiple times, can open up just as easily and turn into a bad group, then back to a good group and vice versa. This is true dispersion. So, if we are conducting a seating depth test merely based on low sample size testing–as in shooting a few rounds per seating depth at only one group and one group appears to look the best, this does not mean that will always produce the same group size. The same thing with a supposed bad seating depth, it can just as easily turn into a good group with higher sample size testing.

four different seating depths shot in random order via blind testing, results at the end of the article. Rapid firing 200 rounds, no cool-down period. Group dispersion from the 0's-.4's. Pretty neat to see how well this competition contour steel barrel from Proof Research has held up under a ton of beating. What surprised me is looking at the POI in relation to where the target is when changing seating depths...

This is why I wanted to test the timing and seating depth method from previously observed results (I used to go to the range, find a good seating depth, come back another day and shoot a bunch...then come to find out the group wasn't always reproducing and it was back to the drawing board) Moreover, to see if the ideology would truly hold up under real-world testing and by shooting large sample sizes like 200 rounds— similar to a match setting.

Nonetheless, a few things should be explained before we go through the test results to understand what is going on that could affect the results.

The rifle recoils first, then the projectile exits second.

Consider a benchrest gun that weighs 30 pounds, that shoots a super low recoiling round. This inherently is going to be a more precise shooting rifle in most shooters hands, than a rifle that weighs 4 pounds, and shoots a super heavy recoiling round. This is because the rifle recoils first, and then the projectile exits second. If you have a rifle that inherently weighs more and shoots a low recoiling round, that will help you shoot more precise groups. To put into perspective how good of shooters you all are, you have to keep less than 0.016” of angular dispersion at the muzzle just to keep a 1 MOA group.


Nonetheless, I wanted to see what it would be like with a shooter behind the rifle as well and test a hybrid style of projectile and do a modified test of what Litz has done. As I am sure many of you are aware, hybrids are advertised as “jump insensitive.” This is important to think about as we go through the results.

Furthermore, if isolating a shooter from this test completely and using a bench rest rail gun, the groups can be wide or super small as well, consistently. We can see this in benchrest rail gun competition results (figure. 1) with the shooter removed. However, that is due to entirely different factors other than what is believed to be a vibrational node that will be discussed in a few months, maybe a year that we will get to show you hopefully on the components side of internal ballistics than if a shooter was on the rifle as well.

To elaborate, there are multiple things that can affect the precision of a rifle system. While some things can be controlled, some cannot be fixed on the reloading bench but rather at the manufacturer and lead us to believe that seeing a result physically is caused by a single variable change on the reloading bench– for example, like just changing seating depth to try and squeeze in groups– but the reason we believe that the result of the group closing in we just yielded, can be due to something entirely different than a seating depth change. Similar to type 1 and type 2 errors in statistics.

Figure. 1 Benchrest rail gun results

The moral of the story, just because we have changed one thing and yield a certain result, doesn't mean that the thing we changed actually caused the end result-especially when only doing small sample sizes, how do we know? What other variables could be affecting the outcome? As an example, there are many things going on internally when changing your seating depth. Is it just seating depth/timing that yields precision? Or could it be what you are doing internally to gas generation by seating deeper? Both? How much does one cause "x" more precision than the other? Is anything else possibly making a bigger difference? This is NOT saying that seating depth doesn’t matter. You have to seat the bullet somewhere in the cartridge case. This is pointing out that there might be more going on to the story than what is commonly believed to yield the precision of a rifle-like a vibrational node or optimal timing that is said to be the leading factor when you change seating by .006". When you seat a projectile deeper; you are changing the rate at which pressure is building and the uniformity of ignition with a given propellant. Finding out how much one variable affects precision is not an easy task with how many different things affect small arms ammunition. However, we could find something that will help us accelerate the load development process through continued testing like this so we can figure out exactly where to seat a projectile even with many things to consider which will save us all time and money.

Recoil Management and Being Consistent

You can have two types of shooters:

Shooter A who is bad at recoil management, but can consistently produce a bad group size with handloads. After all, bad group size is completely subjective based on shooting application. When shooting large sample sizes, it still shows what is truly going on by shooting enough groups to be statistically significant.

Then we have shooter B who uses the same handloads as shooter A, but is very good at recoil management, and conducts the same test. While shooter B is better at recoil management and can consistently produce tighter groups, the conclusion for both shooters A and B, will have the same outcome because both were shooting statistically large enough sample sizes and the results can still be measured.

To elaborate, let’s say before we change anything for the two shooters handloads, we have both of them shoot multiple groups to establish a baseline group size.

Shooter A who is bad at recoil management is consistently grouping 1 MOA on average when shooting 60-3 shot groups

Shooter B who is better at recoil management is constantly grouping ½ MOA on average when shooting 60-3 shot groups as well

Then, we change one variable with the handloads to see If we can statistically observe a change and shooter A goes from a baseline control of 1 MOA group on average to let's say ¾ MOA

And shooter B who is better at recoil management, goes from ½ MOA on average to ¼ MOA. Both shooters are still yielding the same results from the test because they both yielded a ¼ MOA change from the baseline after changing one variable.

Moral of the story, you can still measure a change in precision if you suck at managing recoil or if you are really good at managing recoil which results in increased precision. However, both shooters will still have dispersion of groups that can be measured. Were not all perfect shooters after all!

But, can we factually say that the one variable that we changed truly caused the outcome for the test? How do we know if we only do one seating depth per group, therefore the sample size is only 1? Or is it actually something entirely different that yielded those results that one may not understand? This is type 1 and type 2 errors in statistics. For an example: You go to the doctor to get tested for an illness. There are two errors that could occur.

Type 1 error could be a false positive, meaning the test result says you have the illness but you actually dont.

Type 2 error, which in our example would be a false negative, meaning the test results say we dont have the illness, but we actually do. All in all, things can be very misleading.

Now, let's get down to the results finally…I have failed to prove that seating depth matters with the hybrid style of projectiles.

So, let’s talk about how this test was conducted.

The first thing I did was load up rounds in .006 seating depth increments for four different groups. What I mean by that is I loaded up rounds with .006”, .012”, .018” and 0.024” seating depths from the seating depth I usually shoot which is touching the lands. However, I haven't changed the seating depth throughout the barrels life. When this test was conducted I was about 1600 rounds down through a GT doing rapid firing, with no cool-down periods at all for a full 200 rounds per session.

The lands are being eroded each firing schedule. So, I could be 0.030" off at one point, but, throughout the test, the land measurement is constantly changing with every single round being fired. Measuring erosion-- accurately too, is a daunting task. So, would you think that having a specific "jump" off the lands even matters then if its constantly changing? or, does it have something to do with the seating itself that's altering the burn rate and pressure generation for uniformity of ignition? Both? Something else? Let me know in the comments.

Regardless, before we began, I separated all four seating depths behind my shooting position into four different sections and had someone pick randomly from each pile, 3 rounds specifically of one seating depth, and give them to me to shoot so I did not know what seating depth I was firing to eliminate any bias. This also allowed me to shoot all four seating depths round-robin style in a different order to account for carbon buildup and barrel heat.

While I was shooting, I did not want to break position either so I had her hand me the rounds.

Every single round was recorded and ended up being 190 rounds with a 8.4 SD total out of the 190 rounds in a row. With a magneto, I just learned this the other day unfortunately after shooting 200 rounds and not realizing this, once you get to 99 shots you have to reset it so it continues to record, otherwise it will override your 99th shot with the next follow-up round shot down range. So, I had to re-conduct this test twice to get accurate data, which really sucked.

click arrow to pan to next photo

Moreover, this is a load that I have tested throughout the barrel's life and on any given day, I can confidently say it maintains a 6-8 SD with 90 rounds. I have never shot this load with 190 rounds, in a row recorded, so I was pretty happy. The reason I did not shoot a full 200 rounds was that I completely clean the barrel of all carbon fouling and use the first 10 to get the barrel back to a good condition. The first 10 rounds are always erratic in regard to velocity and need some fouling to get things settled in when cleaning down to the metal.

click arrow to pan to next photo

Seating Depth Test Results

For those who do not know, a regression in statistics works in a way that allows you to look at all of the ways that could possibly influence an outcome. Then, it determines the magnitude and significance of each. In other words, Regression analysis is a set of statistical methods used for the estimation of relationships between a dependent variable and one or more independent variables. It can be utilized to assess the strength of the relationship between variables

When you do a regression of average velocity versus group size from my data (figure. 2), the P-value is .06 which could easily be argued to be significant just on a 94% level instead of 95%.

Figure. 2

Then, just the seating depth p-value is .33, so I’d say that’s insignificant. From a visual perspective, you would expect to see more separation in the boxes that you can see in (figure. 3) if seating depth was making a significant difference in group size in this scenario.

Figure. 3

If you regress both against group size at the same time, seating depth goes to .87 and velocity to .12 (figure. 4). This means my velocity is much more significant for group size than seating depth was.

Figure. 4

When we look back at the velocity regression and seating depth chart (figure. 5): We can see again that velocity had more of a statistical significance than seating depth did in group size. However, we can see that even velocity itself is not super close to the trend line clustered, so something else is going on. This is where type 1 and type 2 errors come into play with other variables. I say this because even though velocity is not the MOST consistent, we can see that seating depth, itself, is less statistically relevant than velocity for group size, which makes this even more important.

Figure. 5

Closing thoughts

Taking that information and looking back at the test results again, it has shown that even with velocities maintaining a single digit SD with 190 rounds recorded, at different seating depths totaling 0.024” difference, the seating depth was not statistically significant to the results of our group sizes.

Shooting this rifle mainly on target for the entirety of its barrel life with the exception of when the barrel was in its infancy stages, remained very consistent in group size even with velocities changing from 2960-2920 FPS because of environmental and barrel wear conditions. In regard to timing, this shouldn't be happening according to the ideology with changes to velocities. Otherwise, the velocity variations would be changing the bullet exit time and result in statistically worse groups.

It could be argued that since the velocities were so consistent over the entirety of this test, it contributed to barrel timing. However, that is not the case. Take for example 2 cars. Car 1 is going 100 mph, and car 2 is also going 100 mph. If car 1 has a 20-foot lead on car 2, that means car 1 will hit the finish line before car 2. The same applies to seating depth, by changing the seating depths, according to the timing theory, you will be altering the barrel lead time each time you alter seating depth and from six thousandths to twenty-four thousandths— they would be all exiting at different times, even with consistent speeds.

To wrap this up, I encourage anyone who has different findings than I do, to please send your chronograph and group data. If you would like to try this for yourself, please make sure to shoot us an email so I can send you the testing requirements. I have already reached out to a number of competitive shooters to conduct the same test and would be very happy if you joined in on the test too. I would really like to add it to what I have and see if we can dig a little deeper into this. If you would like my own data, shoot us an email or drop a comment below, I am more than happy to send it out.

Some personal thoughts: I think we still have a lot to learn about how much, statistically, precision is factually being changed with certain degrees of seating depth. Right now, I think there is more going on than "what meets the eye." Things like barrel flex, accuracy versus precision, recoil management, alignment, dynamic instability of projectiles, only measuring the extreme spread of groups, grain structures of barrels, how barrel heat can affect group size, and measuring actual barrel harmonics in accordance with different contours and when the projectile exits (which has been done thankfully and we will hopefully get to show you in the near future) are not being taken into consideration in most testing and the results are only being attributed to a single variable change; while also conducting the testing with statistically insignificant procedures. What may be true for one rifle could be entirely different for another. Further, there are things that may be getting confused or being contributed to the leading cause of precision, when in actuality, it could be a magnitude of things and we might be "cherry-picking" statistically insignificant data in regard to how one would commonly approach a seating depth test. To say seating depth as a single variable, or something else leads to precision more than another would be "jumping the gun" a little early, especially when only testing a single group per individual seating depth in commonly practiced load development methods. To prove something takes a great magnitude of testing and data, which I hope to try and do so we can continue to shoot and focus on things that matter the most.

Thank you for reading! If you are interested in learning how to reload or are an experienced handloader looking to learn how internal ballistics works to significantly cut down on load development time you can find our classes here

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