Seating depth - does it even matter?

[Andouille]

My seating depth test with .223 65gr Sierra SBT. Seating depth starting at .010” off lands and in increments of .003” with 5 shot groups.

My takeaway is the bullets seem to prefer to be closer to the lands, but not definitive yet.

I have a question for your consideration and to better understand your test rationale; would measuring accuracy by changing seating depth in minute increments within the range of appreciable throat erosion require you to continually chase the lands? A throat erosion can be 0.003"-0.006 per 100 rounds for popular 6-6.5mm calibers as discussed in this Precision Rifle Blog article.

What if all that is needed for seating depth is to find the broad "flat spot" in the dispersion and seating within that depth to negate effects of throat erosion and avoid chasing the lands? There's a great discussion within this article: Bullet Jump & Seating Depth: Best Practices & Conventional Wisdom.

 

[Plainsman-147]

Not perfect, but here's the 3 minute attempt at overlaying those two sets of 5 shot groups into one set of 10:
View attachment 995918
Much less different now. Can't really pull a measurement from these, but to my eye there is very little difference, and probably even less of an obvious trend. Proper 10+rd groups assessed with shot radius and the T-Test would say for sure.

Wow I’m impressed with your editing skills. But I think your original interpretation of my test was correct. I shot 5 rounds at each seating depth (total of 10 different seating depths ranging from .010 off lands to .037 off). Sorry for the confusion

 

[Plainsman-147]

I have a question for your consideration and to better understand your test rationale; would measuring accuracy by changing seating depth in minute increments within the range of appreciable throat erosion require you to continually chase the lands? A throat erosion can be 0.003"-0.006 per 100 rounds for popular 6-6.5mm calibers as discussed in this Precision Rifle Blog article.

What if all that is needed for seating depth is to find the broad "flat spot" in the dispersion and seating within that depth to negate effects of throat erosion and avoid chasing the lands? There's a great discussion within this article: Bullet Jump & Seating Depth: Best Practices & Conventional Wisdom.

You bring up a great point. With my test, my measurements showed that across the top 4 groups, the group sizes were roughly .75” or less. So if I pick a seating depth in the middle of that .012” window then I would be good for 300-400 shots of throat erosion before needing to lengthen my rounds to stay in the “acceptable jump” window.

Please correct me if my rationale is wrong. I attached a better picture of my test. My takeaway from the test was that the groups seemed to open up around the .022 jump range. Also with the assumption that 5 round groups are sufficient enough of a sample size (which may not be true)

 

[Harvey_NW]

Also with the assumption that 5 round groups are sufficient enough of a sample size (which may not be true)

This, IMHO. There's statistically about 50-60% variability in 5 shot groups, so the same load could print wildly different, depending on the precision capability of the rifle and load combination. A rifle that averages 1" for 5 shots could print a .4" group or a 1.6" group, with the same load. Granted, you shot a decent sample size for the overall test, I see the same thing as @solarshooter with the overplayed groups. Really not all that different, each sample is just random distribution that falls within the true cone of fire.

I appreciate your contributions to the thread!