Thread: Seating Depth Test. Waste of Time?

I think there are a couple of different ways you might look at this. Here's my thinking right now.


The process generally followed for load development (which it appears you are following, and which I also followed for many many years) is basically an attempt at optimisation, rather than working to meet a specific objective. Running a series of tests where you control different variables, with the hypothesis that changing these variables changes the load characteristics, and that you can use the test data to select the best combination of variables to get the best load, defined as the load expected to produce the best precision possible (or lowest velocity SD, or both), without regard to requirements.

The tests we use however are flawed for selecting the load with the best precision (or lowest velocity SD), when the differences are small or moderate. From comparing single 3 or 5 shot groups, we do not get sufficient information to state which combination gives the best, or the worst, result - or to confidently identify whether there is a difference. From the selection of 4-shot groups above in your seating depth test, I am unable to identify whether there is any difference between them or not.


It is probably prohibitively time-consuming and expensive to really identify which is the best combination - it may not be a practical approach to look for optimisation in this way. The differences may be non-existent, or insignificant in a practical sense even if they can be identified through a more robust testing process.


An alternative is to take an objectives-based approach to developing a load. What are your requirements? Define these, then if a load meets your requirements, you are done. This however requires some re-framing around the definitions of your requirements compared to common usage.

What are the criteria for requirements? They will vary depending on your specific use, but I would think to start (once you have selected the appropriate components most likely to give the result you want):

- Sufficient precision from the system that, excluding shooter proficiency in the field and weather/ballistic effects, hit probability is high for the targets/ranges I require the rifle to perform at. For a big game hunting rifle, used at conventional distances, this might be that you need the rifle to consistently (say 95% of shots) be capable of hitting a 50mm circle at 100m - a "2moa" rifle. This is a realistic expectation for a hunting rifle when you get a genuine measure of precision.

- Velocity within your required range. this is likely defined by what you need the projectile to do. Setting requirements for this does involve being sensible at the time you select your cartridge. You will not get sufficient speed from a .223 with an 18 inch barrel to have a 80gr ELDM expand at 700 metres. You will get sufficient speed for it to expand at 400 metres. For this you require 2550fps at the muzzle. If your requirement is to shoot deer at 400 metres with your .223 and the 80gr ELDM, your velocity requirement is 2550+ FPS at the muzzle. Increases in velocity result in only vanishingly small improvements in hit probability; so the main sensible reason I can currently identify to be seeking velocity is for projectile performance on target.

- Consistent velocity - yeah your ES and SD. It's worth being aware of how much this really matters to set your requirements - for conventional hunting - you can more or less ignore it. For very long ranges it becomes significant. It is not clear that there is anything that can be done with load tweaking to improve it, if you are using good components and have a good reloading process with quality equipment set up correctly, it is what it is.

Hit probability is a subject in itself worth examining, one of Bryan Litz's books is gathering dust on my reloading bench and contains quite a comprehensive examination of the subject. I will have a look at which one so the discerning reader can find out more.

This testing can be done at a much coarser scale than making small tweaks to powder charge/seating depth. If, with a valid measurement, a combination in a .223 does not shoot precisely enough for your requirements at 24.5 grains of 2206H it will not at 25.0 grains. Try a different combination (powder, bullet, etc). It may shoot better at 22.0 grains than at 24.5, but if that is outside your desired velocity window it is irrelevant to test it.


I have not specifically tested small changes in seating depth to test whether there is a change in precision. Within your functional window for your requirements there are limits - generally for practical use we need to seat shorter than mag length, and some distance off the lands, while keeping our ogive outside the case. Generally in practical terms seating something around .020" off the lands as a starting point seems to give precision that meets requirements as long as it fits in your magazine. It may be that there is a difference in precision between seating .020" off, vs .500" off - with the expectation that the shorter jump would be more precise - however it's not clear why you'd ever load .500" off if there was a choice. To quote Miles Neville from Hornady - set your OAL .030" off the lands and forget your seating die is adjustable. This to me has been a practical solution.


This gentleman presents some interesting information on the validity of selecting loads based on small tweaks to seating depth and the difficulty of meaningfully identifying differences - https://www.youtube.com/watch?v=_h4iAiGYbwg



All the theory in the world about harmonics is wonderful, but my interest is - what is the most efficient way to get a realistic and practical understanding of whether a load meets requirements?