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.
Seating depth - does it even matter?
- Thread starter Harvey_NW
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pbroski
Lil-Rokslider
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.
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'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?
wind gypsy
"DADDY"
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Any load that is that sensitive about 3 or 6 thou seating depth can eff right off. That is wild.
pbroski
Lil-Rokslider
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.
pbroski
Lil-Rokslider
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.
pbroski
Lil-Rokslider
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.
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.
pbroski
Lil-Rokslider
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.
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.
pbroski
Lil-Rokslider
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.
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.
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.
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.
JGRaider
WKR
Lot's more variables are introduced at 670yds than seating depth, IME.
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!
wind gypsy
"DADDY"
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Hey man, dont be making us question our minimal load testing nirvana with your data!
, like a 10x scope, shooting sticks,Atv tire pressure and the pack. Not to mention the environmentals
solarshooter
WKR
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.
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.
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.
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.
wind gypsy
"DADDY"
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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?
TaperPin
WKR
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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
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
solarshooter
WKR
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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