Seating depth…what does it do?

[David Christian]

I started this thread in the search of knowledge and a way to find an absolute with a specific part of a process. I’ve learned a number of new angles to pursue out of curiosity. Good stuff!

I am a big believer in the frictional process of debate. Everyone gets to postulate and defend their idea as well as hear, interpret and then either accept or debunk others’ ideas. This thread is a good example.

I am still left wondering if anyone on this forum has truly figured out seating depth? Is there a predictive model? Like many of the posts, I am pretty good at trial and error, that is until a variable changes, such as ambient temperature, lands length, rounds down the barrel, dirtiness of the barrel, etc.

Is trial and error periodically throughout the barrel’s life the best route?

DC

 

[BoydAllen]

Seating depth would change
1) Effective case capacity
2) Time between initial ignition and time when the bullet engages the rifling
3) Initial bullet speed when said bullet engages the rifling
4) Initial case pressure rise and peak pressure
5) Bullet barrel time +/- time in 2)

Years ago, I changed my seating depth for a load, seating it deeper into the rifling by .002, but I guessed wrong and it threw my load out of tune. I guess my point is that with such a small move, and the change I observed, I think that the actual mechanism of the change in accuracy will always be no better than a guess. Frankly, I think that our time would be better spent experimenting and taking good notes of what happens when we make a particular change, be it neck tension, charge weight or seating depth, and it would be a lot more honest to simply say that we don't really KNOW what does it but we can know what the effect is. Some of these threads remind me of pre science explanations of the night sky.

 

[Beiruty]

I started this thread in the search of knowledge and a way to find an absolute with a specific part of a process. I’ve learned a number of new angles to pursue out of curiosity. Good stuff!

I am a big believer in the frictional process of debate. Everyone gets to postulate and defend their idea as well as hear, interpret and then either accept or debunk others’ ideas. This thread is a good example.

I am still left wondering if anyone on this forum has truly figured out seating depth? Is there a predictive model? Like many of the posts, I am pretty good at trial and error, that is until a variable changes, such as ambient temperature, lands length, rounds down the barrel, dirtiness of the barrel, etc.

Is trial and error periodically throughout the barrel’s life the best route?

DC

There is no secret about internal ballistic of a bullet.
Whatever variable we consider:

Primers
Powder speed
Powder charge
Case volume
COAL
Bullet profile
Bullet weight
>>> Bullet seating depth
Case neck tension
Bullet caliber
Chamber profile
Barrel rifling
Barrel length
Barrel profile

would change:
Pressure behind the bullet
Bullet acceleration and speed
Bullet barrel time
Muzzle Velocity

As for the implication on accuracy, the major factors that would define the precision of the bullet/rifle are:
Barrel profile, (i.e, heavy 1.2" barrel vs ultralight thin barrel)
Overall weight of the rifle
Rifle build quality

All other factors are minors. When major factors are sub par, the minor factors would play more role in the accuracy of the rifle/bullet. "Tuning" would start getting useful in enhancing the accuracy/precision.

All what you need to know about bullet seating depth

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[Kracken McGraw]

Thanks for this very interesting thread. It sure confirms for me that a test needs to be done at the distance range that I plan on shooting at.

 

[Jager]

Yes it matters. Here's what it looks like in pictures. Two-shot groups.

 

[Bc'z]

Yes it matters. Here's what it looks like in pictures. Two-shot groups.


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Excellent example of how seating nodes repeat coming in and out of tune.

 

[Kracken McGraw]

Yes it matters. Here's what it looks like in pictures. Two-shot groups.


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I was curious if you go straight to the one hole group, or seat out past it a little for throat erosion.

 

[Liven The Dream]

Neck Tension when my load got close to my target velocity .
Working up a load for my .284 F/Class Rifle with A-4350 !
The different things in play are A-4350 and .284 Lapua Brass ( not 6.5 necked up.
I was doing my standard .015 jump with 180 Berger HB. got up to my target Velocity 277??.
SD was 3.6 5 shots. The grouping was so so ? I get home whats up with this, checking all my steps ?
I come up with same Bushing in Die I used with 6.5X284 necked up Brass ?New .284 Brass thicker necks.
Change Neck Bushing .001 Larger, load test Rounds ( only Change Bushing ).
Five shots at 200 yards covered with 3/8 Target Dot, 2771 AV. SD 4.1 Neck Tension .002.
Now Weather get Better can this get a repeat ? I sure hope so .

Hope This is of some Value

 

[Jager]

I was curious if you go straight to the one hole group, or seat out past it a little for throat erosion.

No, I don't try and second-guess anything else going on with the rifle. The target is the ultimate arbiter of truth for me.

But what is important is that there be a pattern. A single, nice group doesn't cut it. You have to be able to see the tune come in and go out. If you can't see that, then you've still got work to do...

 

[Rtheurer]

Different powders "like" different amounts of neck tension. What powder were you shooting? What caliber?

Mr Boyd.
I was wondering if you have witnessed what i have. Seems like when i run a Faster ( for a givin cartridge) powder vs a Slower burn rate of powder that I end up with better results using MORE neck tension with the faster powder and less neck tension with the slower burn rate of powder. So far that has been my experience when doing a neck tension test.
I was wondering if you have seen that same thing or opposite of what i have witnessed.

 

[BoydAllen]

Mr Boyd.
I was wondering if you have witnessed what i have. Seems like when i run a Faster ( for a givin cartridge) powder vs a Slower burn rate of powder that I end up with better results using MORE neck tension with the faster powder and less neck tension with the slower burn rate of powder. So far that has been my experience when doing a neck tension test.
I was wondering if you have seen that same thing or opposite of what i have witnessed.

When dealing with a new powder, generally I am loading at the range, and I have the means with me (neck bushings) to experiment with neck tension, so I do. It all gets back to testing everything and believing your targets. Getting to the specifics of your question, I have seen it go both ways, so I do not operate on that particular assumption.

 

[GetReal]

My observation / experience is seating depth (1) tightens up groups and (2) helps eliminate that 5th round "flyer" (not caused by shooter error.

Berger told me:

1. Start with a low end powder charge.
2. Do a seating depth test to find what's best. For Hybrid targets, start test at 15 / 30 / 45 / 60 off the lands. And dial it in from there + / - 5 thou.
3. Work up powder charges to get the velocity you want.

 

[Dusty Stevens]

I was curious if you go straight to the one hole group, or seat out past it a little for throat erosion.

Id rather it shoot now than maybe next match

 

[Kracken McGraw]

Id rather it shoot now than maybe next match

Lol, I bet it might help you win when you find it.

 

[kthomas]

With all of your years as a test shooter for manufacturers, are you willing to share your knowledge and experience? I’m seriously curious to hear your opinions.

I'm not a test shooter for manufacturers, you must have me mistaken for someone else.

 

[kthomas]

Yes it matters. Here's what it looks like in pictures. Two-shot groups.


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Classic example of using sample sizes that have no statistical relevance in which we can draw conclusions from.

I don't mean this to offend, it seems all reloaders are guilty of this. As a community, we have an abhorrent understanding of statistics and statistical analysis, which leads to very flawed conclusions.

 

[Jager]

Classic example of using sample sizes that have no statistical relevance in which we can draw conclusions from.

I don't mean this to offend, it seems all reloaders are guilty of this. As a community, we have an abhorrent understanding of statistics and statistical analysis, which leads to very flawed conclusions.

No offense taken.

Statistics are relevant in some places. Not in others.

When I'm running a seating depth test I'm not at all looking for statistical confirmation. I'm mostly looking for all those loads that don't work - which is most of them.

Two shots is ridiculous from a statistical standpoint, of course. But once that second shot has edged out away from the first, the group isn't going to get any smaller. And I don't need to see any more.

Which is not to say that once I see a two-shot "group" that look promising I call it a winner and call it a day. The load(s) that look promising will most certainly get their chance to prove their repeatability. Or not.

 

[Acyr]

Classic example of using sample sizes that have no statistical relevance in which we can draw conclusions from.

I don't mean this to offend, it seems all reloaders are guilty of this. As a community, we have an abhorrent understanding of statistics and statistical analysis, which leads to very flawed conclusions.

What if he could replicate this test multiple times? Would it matter then? All reloaders are not guilty of this. As a statistical expert you should know better than to draw those conclusions.

 

[kthomas]

What if he could replicate this test multiple times? Would it matter then? All reloaders are not guilty of this. As a statistical expert you should know better than to draw those conclusions.

I'm certainly not a statistical expert (I seem to get mistaken for someone else on this specific forum frequently).

Replication is key. If those tests were conducted 10, 20, 100+ times with identical results, that would certainly be a better data set to draw conclusions from. There are certainly limitations to the test however, such as the degree of precision to which the seating depths really are, taking into account any error of the operator and the degree of precision of said calipers, amongst other things. There's also limitations to how the testing is done, specifically from the shooter side - there'll certainly be induced error, which would be impossible to parse out with such small data sets.

And this hypothesis of bullets coming into and out of "nodes" - what is the explanation for that? What is the mechanism that allows that happen? There seems to be a lot of conjecture around what the projectile is doing and what's influencing its behavior. No one is quite able to really explain that. I would love to see some detailed and scientifically conducted tests on this, rather than the observational/anecdotal data of low quality that we always see.

All that said, I would be very interested in seeing this test duplicated many times over, to see if the results repeated themselves consistently.

 

[gunsandgunsmithing]

I haven't chased lands in a bbl since I started using tuners in about 2007. That should answer the question a lot of people have here...."in bore time"...or exit time relative to muzzle position...That's tuning in a nutshell.