Seating depth - does it even matter?

The first group is 10 shots with a cbto of 1.480". The second is 9 shots at cbto of 1.483" and the third is 9 shots at cbto of 1.486". All shots were fired round robin style.

Pretty interesting results, I would say.
Interesting to say the least. This is the first time I've seen that much variance with decent sample sizes, especially with such little change. Thanks for shooting the shots and posting your results.
 
It's the greatest variance I've seen as well. I've done a decent amount of testing recently and I believe seating depth makes a measurable difference in at least some gun/ammo combinations.

The main test rifle is a .223 Howa with the factory Stocky stock topped off with a Nightforce NXS 2.5-10x42 firing 73 gr. ELD-M with H4895.

I've done 10 shot strings at 100 yards and 825 yards and I'm generally seeing about a 25 percent improvement between a good cbto vs. a bad one. If I get time I might post some more results.
 
How many more
I did a seating depth test today with my CZ527 American in .17 Hornet. The scope is an old Leupold Vari X III 6.5-20x40 EFR.

The load is Hornady brass, Hornady 15.5 gr NTX bullet, CCI 400 primer and VV N120 powder. Velocity is approximately 3975 fps. The range was 99.7 yards fired off of a front and rear sandbag rest.

The first group is 10 shots with a cbto of 1.480". The second is 9 shots at cbto of 1.483" and the third is 9 shots at cbto of 1.486". All shots were fired round robin style.

Pretty interesting results, I would say.

BTW, it was my first time using my new Garmin chronograph. It picked up every single shot where as the LabRadar I used previously had not picked up even a single shot. I'm very impressed with the Garmin.
I'm curious if it has something to do with the case volume. 17 hornet is a pretty small cartridge compared to say a 300 Wm. That 0.006 difference as a % of case volume of the hornet vs a Wm would be quite the difference no? I haven't done the math but seems like it could be enough?
 
Yes it's small but is still a bottleneck cartridge. The bullet diameter/case capacity ratio is on par with many standard size cartridges. According to QuickLoad a change of .006" of seating depth will only change the velocity by about 5 fps, so not much difference.
 
I would check .003 the other way from the top target. If it doesn't shoot decent, I would test the good load again. If it is really that finicky, I would be interested to see the reamer print.
 
That's fine. But if there was that much gain in precision to be had, I think a lot of other people would take it. I know I would.
I did leave the post open ended, but I'm still taking your results with a grain of salt. I would assume that such a small caliber bullet with very short bearing surface could be subject to something like in bore tilt with farther jumps to the lands, especially if there was extra clearance in the throat. Like mentioned above, it would be interesting to see the reamer print and know the dimensions.

This combination might actually be a case where seating depth does matter, but I would question the consistency of it lasting for long, or being a "chase the lands" and always tweaking situation.
 
It's a factory barrel, so I don't have a reamer print. The .17 Hornet is a relatively new creation by Hornady, so the tolerances are likely pretty tight. What I can tell you is that at a cbto of 1.486", the bullet is only about .008" from the lands, so not much room for tilt.

I would load at 1.482". It might be a few hundred shots before it wears itself out of the node. I'll monitor throat erosion to actually see what happens.

I'm also going to test some more loads, going shorter in length.
 
You all might find this interesting. I did load development as described in post 29 of this thread https://rokslide.com/forums/threads/how-far-off-pressure-are-you-loading.409505/page-2

First is a Howa .223 Carbon Stalker factory rifle with 2.5-15 Credo. Load is 73 gr. eld-x, Benchmark powder, Lapua brass and CCI400 primer. Did a seating depth test at 587 yards. Shot seated with sticks and pack leaning against my ATV. I fired 10 rounds at 7 seating depths .003" apart. Shot round robin.

Of those 7, I took what appeared to be the biggest and the smallest and did a 30 round seating depth shoot-out between the two. Here is what it looks like. On the left is CBTO 1.925" and the right CBTO 1.917" On the bottom are the 10 round groups and at the top are their corresponding 30 round groups.

The left is 30 @12.75" or 2.075 moa. Vertical is 9.59" or 1.560 moa.

The right is 30 @ 10.75" or 1.749 moa. Vertical is 4.89" or .796 moa.

IMG_20250525_191713~2.jpg
 
Here's another one. The rifle is a custom Kimber .22arc with 2.5-10 NXS shooting 80 gr. eld-m, Varget, Lapua brass and CCI450 primer. This one was shot at 657 yards seated from sticks and pack, leaning on ATV. Shot round robin.

On left is CBTO 1.743" and right is 1.747".

Left is 30 @ 6.74" or .956 moa. Vertical 4.87" or .708 moa.
Right is 30 @ 13.07" or 1.900 moa. Vertical 11.59 or 1.685 moa.

IMG_20250524_080647.jpg


IMG_20250526_214638~3.jpg


To me, it looks like seating depth does matter. And the nodes are close together just like the top shooters say they are. Guys like Cortina, Boyer, Neary, Glasscock, etc. But anyway, I'm going to keep testing. It's fun stuff.
 
Here's another one. The rifle is a custom Kimber .22arc with 2.5-10 NXS shooting 80 gr. eld-m, Varget, Lapua brass and CCI450 primer. This one was shot at 657 yards seated from sticks and pack, leaning on ATV. Shot round robin.

On left is CBTO 1.743" and right is 1.747".

Left is 30 @ 6.74" or .956 moa. Vertical 4.87" or .708 moa.
Right is 30 @ 13.07" or 1.900 moa. Vertical 11.59 or 1.685 moa.

View attachment 895007


View attachment 895009


To me, it looks like seating depth does matter. And the nodes are close together just like the top shooters say they are. Guys like Cortina, Boyer, Neary, Glasscock, etc. But anyway, I'm going to keep testing. It's fun stuff.
Man, that's some dedicated testing! I can definitely appreciate the amount of work put into this, but as mentioned above there are a lot of variables introduced when shooting a 22 cal 657 yards with sticks and a pack leaned up against an ATV. Regardless, I appreciate the input. Good shooting!
 
Here's another one. The rifle is a custom Kimber .22arc with 2.5-10 NXS shooting 80 gr. eld-m, Varget, Lapua brass and CCI450 primer. This one was shot at 657 yards seated from sticks and pack, leaning on ATV. Shot round robin.

On left is CBTO 1.743" and right is 1.747".

Left is 30 @ 6.74" or .956 moa. Vertical 4.87" or .708 moa.
Right is 30 @ 13.07" or 1.900 moa. Vertical 11.59 or 1.685 moa.

View attachment 895007


View attachment 895009


To me, it looks like seating depth does matter. And the nodes are close together just like the top shooters say they are. Guys like Cortina, Boyer, Neary, Glasscock, etc. But anyway, I'm going to keep testing. It's fun stuff.

Hey man, dont be making us question our minimal load testing nirvana with your data!
 
Yeah, there's a reason testing is typically done at short range. Environmental effects will add significant dispersion to your groups at longer ranges, especially over a 60rd course of fire. You also have barrel heat, and shooter fatigue to consider.

Now, 30rd groups are certainly more statistically robust and useful for distinguishing small differences, but they need to be 30 "perfect" shots, as in no other variables changing during the courses of fire. Very hard to achieve that level of consistency without machines and indoor ranges, which is why that type of testing is usually done with machines and indoor ranges!

Once again I'll advocate for using shot radius, mean shot radius, and the T-test to more rigorously assess different samples. This post has an example of how I do this: https://rokslide.com/forums/threads/what-did-you-do-in-the-reloading-room-today.338563/post-4125326.
To recap it all, here is a summary of tested combinations and results:
LoadVariable being tested10-shot Mean Radius % of BaselineT-test vs Baseline
57.0gr IMR7828SSC, 162gr ELDM, CCI200, 0.060" jumpBaseline0.55"--
57.0gr IMR7828SSC, 162gr ELDM, FED210, 0.060" jumpDifferent primer1.04"189%0.00
55.0gr H4831SC, 162gr ELDM, CCI200, 0.060" jumpDifferent powder #10.33"60%0.02
50.0 H4350, 162gr ELDM, CCI200, 0.060" jumpDifferent powder #20.45"82%0.18
55.0gr IMR7828SSC, 162gr ELDM, CCI200, 0.060" jumpDifferent charge weight0.64"116%0.29
57.0gr IMR7828SSC, 162gr ELDM, CCI200, 0.020" jumpDifferent seating depth0.72"131%0.12
55.0gr IMR7828SSC, 175gr EH, CCI200, 0.060" jumpDifferent bullet0.43"78%0.15

My conclusions are that changing primer and changing powder are the only things that made a significant difference (>95% confidence) to precision. One of the two powder changes, a major seating depth change, and a bullet change, all had >80% confidence of being significant changes, and qualitatively I'm guessing would show a higher confidence difference given more samples. Changing charge weight by 2gr (3.5%) had the least significant effect on precision.
You'll note in that test I did not see any significant effect from seating depth, and I made a very drastic change with a secant ogive bullet.

I do think that seating depth can matter for loose chambers with long freebore, where the bullet is jumping truly unsupported for a significant distance before engaging the lands and can develop yaw before engraving which will manifest as increased dispersion on target. For instance, a SAAMI 308 chamber has a 0.310" freebore, meaning there is 0.001" clearance around the bullet on all sides. Compare this to a 6.5CM SAAMI chamber with a 0.2645" freebore, or 0.00025" clearance around the bullet on all sides, effectively a light interference fit. This will constrain the bullet much more tightly and prevent it from developing any appreciable yaw even with a very long jump. So you might find different results testing in different chambers with different bullets and jumps.
 
💯 , like a 10x scope, shooting sticks,
Atv tire pressure and the pack. Not to mention the environmentals

Looks like 4 of the 30 on the larger group fell out of the distribution of the smaller group. And if he shot them round robin, seems unlikely that something went wrong for all 4 on one group but not the other?
 
Trying to coax absolute truths out of reloading practices, especially with the wild variability between factory rifle barrels, will usually be an exercise in futility. It’s good to see a lot of deep thinking on the subject to make sense out of it, but like many issues we try to solve in our head, actually burning powder resolves the question quickly for our loads in our rifles. It’s a lot of work to fine tune seating depth, and I don’t feel the juice is worth the squeeze, despite all the articles and books that might mention it. A friend loves to tinker and 90% of his shooting is playing with variables like this and there’s nothing wrong with that kink, but I’d rather blast away at rocks, or prairie dogs, or plates.

If there was one magical range of seating depths every rifle made would be throated for one specific load, which of course they are not. Even with the highest quality barrels and most particular reloaders in benchrest competition with 30 guys sporting the same cartridge, not everyone is shooting into the lands or the same distance off, but not a single one will be barely touching the lands because that is a known universal bad practice.

You burn off .0001” or so of the lands every shot, so if you don’t like a distance of the lands, stick around and it will change. lol
 
All of the theoretical and experimental study I've done has only led to me tinkering less. 99.5% of reloaders are wasting their time and money, ESPECIALLY competition and benchrest shooters. The best equipment, components, and consistency in the reloading room and as a shooter are the real "secrets" to accuracy.
 
All of the theoretical and experimental study I've done has only led to me tinkering less. 99.5% of reloaders are wasting their time and money, ESPECIALLY competition and benchrest shooters. The best equipment, components, and consistency in the reloading room and as a shooter are the real "secrets" to accuracy.
Agreed. Find a bullet and powder the barrel likes, and IME in a properly setup rifle it will tell you pretty quickly.
 
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