Seating depth theory

[Murray Brook]

But our ability to model internal combustion and dynamics is either government secret or does not yet exist.

I don't believe there could be a formula for this. Just think about it. No two rounds are 100% alike no matter how careful you reload. Add to this that every round fired in the barrel does so at a different temp as well as a different degree of fouling and each barrel does not foul to the same extent. Then there is humidity and many other factors. I just can't see a one fits all formula. JMO

 

[dgeesaman]

I don't believe there could be a formula for this. Just think about it. No two rounds are 100% alike no matter how careful you reload. Add to this that every round fired in the barrel does so at a different temp as well as a different degree of fouling and each barrel does not foul to the same extent. Then there is humidity and many other factors. I just can't see a one fits all formula. JMO

Combustion modeling is typically done using explicit fluid dynamic methods, which is essentially millions of equations stacked on top of each other. So when I suggest there might be Engineering methods, I don’t expect it to come to a single formula with a handful of terms. That would be nice but exceptionally lucky.

 

[Shooter13]

I don't really think that there is a why. Some people prefer pizza others burgers. I've have rifles that shoot their best at touch and one that shoots great at only .120 off. Just preference I guess.

You're right, it is preference for each gun, how do you know what each gun likes? TESTING!! It's like all the talk of tuners and just randomly pick a number or adjust the tuner every 3 or 4 marks, if you don't test EACH setting you are not going to know.

 

[mikecr]

Do we think that I could arbitrarily choose a powder charge [223] and tune with seating depth to get 1/2 MOA?

What I'm suggesting is that seating depth is far coarser to results than powder changes.

The last barrel I developed with provided solid 3/8moa with 300y testing. I could vary the powder any amount any direction and get no worse than 1/2moa. But change seating 30thou from my optimum, and I could see 1moa or worse.
If you're serious about accuracy, you need to do FULL seating testing.

Now, If i had just pulled the 30thou bad out of my butt to begin load development with, would I have reached 3/8moa with powder alone? I don't know but I doubt it. I doubt the 3/8moa would be reliable.
And since optimum seating is independent of powder, I figure it's better to find best seating before moving to powder testing. That is, as prerequisite to load development.
Same with primer swapping/testing. Attempting that from a tune, to learn anything, is wasted effort.

 

[Bottom Fodder]

What I'm suggesting is that seating depth is far coarser to results than powder changes.

The last barrel I developed with provided solid 3/8moa with 300y testing. I could vary the powder any amount any direction and get no worse than 1/2moa. But change seating 30thou from my optimum, and I could see 1moa or worse.
If you're serious about accuracy, you need to do FULL seating testing.

Now, If i had just pulled the 30thou bad out of my butt to begin load development with, would I have reached 3/8moa with powder alone? I don't know but I doubt it. I doubt the 3/8moa would be reliable.
And since optimum seating is independent of powder, I figure it's better to find best seating before moving to powder testing. That is, as prerequisite to load development.
Same with primer swapping/testing. Attempting that from a tune, to learn anything, is wasted effort.

Mike did you start at jam and back out say .02,.04,.06 etc. until you found that sweet spot at 30? Just wondering how much most people adjust each step
thanks for sharing

 

[ORjeff]

Every gun I've worked with (probably not as many as most of you have worked with) has shown a sensitivity to bullet jump. In my limited experience there is a definite sweat spot that is .005"-.010" wide centered in the neighborhood of .050" (varies between gun/bullet combo). Like many of you my experience has convinced me that bullet jump matters and like many of you I've pondered why.

Since you don't know me I should tell you that I am a physicist/engineer/professional experimental scientist. The question of why bullet jump matters fascinates me to the point of obsession and is the singular reason I became interested in precision reloading. That fascination has driven me towards a new solution set that you'll find at superior-precision.com, by the way. I'm also a machinist/mechanic with a lifetime of real world hands-on experience working with metals, etc. So, as for why bullet jump matters, I have spent a lot of time thinking about this. As a professional experimental scientist I've also spent a lot of time pondering how to explore this in a way that would provide clear answers to the question but that would require resources beyond my means. Not to be discouraged I've been left with whatever this fatty blob between my ears can imagine. Here are my thoughts....

I suspect that it's all about the velocity of the bullet when it engages the lands. As the bullet engages the lands it is deformed, some say engraved. Some of you are machinists. You know that cutting speed matters, you know that the surface finish you get depends on surface feet per minute (velocity of the material at the cutting edge). If you go too slow and the surface looks like crap as the material tears at the cutting edge. Increase the speed and you find a point where the surface finish improves because the material is now fracturing at the cutting edge instead of tearing. Go too fast and you burn up the cutter and the material. I suspect that the same thing is happening here but the result will depend upon the bullet (jacket design, etc) and the rifling (5R vs square, etc) and I suspect that some combos are less sensitive that others. I suspect that how the jacket deforms is very dependent upon the velocity; if too slow it deforms unevenly causing the bullet to drift off center, if too fast then other things start to happen.

When the primer explodes (unlike the powder which burns), the resulting very rapid rise in pressure is more that enough to push the bullet into the lands which probably happens before the powder ignites. That pressure is accelerating the bullet. Start with the simplifying assumption of constant pressure during this time period therefore constant acceleration. Per classic equations of motion V^2=2ax. Double bullet jump (x) and V increases by a factor of sq.rt of 2 which is 1.4x. Go from .005 to .050 and you've changed jump by a factor of 10 so the velocity of the bullet when it hits the lands increases by a factor of sq.rt of 10 = 3.16x. The simplifying assumption is probably incorrect so the actual increase in velocity if probably much higher. Anyway...the important part of the thesis is that if the bullet hits the lands too slowly the jacket material tears/deforms in an irregular way. Go fast enough and the deformation (engraving) is more consistent.

However, increasing bullet velocity as it enters the rifling also increases the angular acceleration as the bullet goes from zero rpm to whatever it needs to be to move forward. Double the velocity and you've doubled the angular velocity that the bullet needs to suddenly achieve which doubles the forces between the rifling and the jacket. At some point the jacket material will not support that much rotational acceleration and it will start to tear.

So, I surmise (without any direct evidence to support it aside from a lifetime of working with metals) that if the velocity of the bullet when it hits the lands is too small the jacket undergoes non-uniform plastic deformation which tends to result in the bullet becoming non-concentric and if it's too high the jacket undergoes non-uniform brittle deformation which tends to result in a non-uniform jacket surface when it leaves the barrel. Both cause irregular flight. Somewhere in the middle there is a sweet spot. Data to test the thesis would require some fairly expensive equipment which I'd love to play with but don't have so I stay focused on the pragmatic reality that why bullet jump matters doesn't matter. I do the load development work req'd to find the sweet spot then rely on my seater die to consistently give me the bullet jump I need.

 

[jimmymac]

I have not jumped a lot of bullets in the short range game. I’m in the lands to some extent pretty much always. I’ve experimented with jumping bullets but always come back to being in the lands.

 

[boltfluter]

I have not jumped a lot of bullets in the short range game. I’m in the lands to some extent pretty much always. I’ve experimented with jumping bullets but always come back to being in the lands.

My results mirror yours Jimmy. Sometimes the results are shockingly shocking!
Paul

 

[mikecr]

Mike did you start at jam and back out say .02,.04,.06 etc. until you found that sweet spot at 30? Just wondering how much most people adjust each step

I didn't say I found best at 30thou OTL. I said a change of 30thou from my optimum.
I do very close to Berger recommended full seating testing. Coarse, and then finer (while away from any powder tune), and then finest within a seating window (group shaping after powder development).

I have not jumped a lot of bullets in the short range game. I’m in the lands to some extent pretty much always. I’ve experimented with jumping bullets but always come back to being in the lands.

This is how point blank BR shooters concluded that bullets had to be in the lands(ITL) to shoot well.
But that was rarely true outside of a tiny underbore.

The problem: PB BR shooters did not actually do correct full seating testing, as it was useless for their particular cartridges, which rely on a high starting pressure that seating ITL provides.
Their little wiggles while ITL amounted to no more than group shaping, and leaving land contact collapsed their tunes completely. So that left them thinking that seating was a fine tune from an incredibly limited place,, and that's true -with a 6PPC.
It's one of several areas where PB BR proves itself as non-applicable to the rest of the shooting world.

Now, thanks to the declarations of PB BR shooters (for many decades), most people still think seating is tuning, and that it's fine tuning. When someone with a hunting cartridge chooses not to be ITL, they pick seating as pulled out of their butts or book, and go into powder testing with it. Then, from a powder tune, they try seating testing. But of course changing seating from tune will collapse their tune at the same time as changing seating results in itself. Classic tail chasing.
It's all backwards, all wrong.

 

[Fuj]

Now, thanks to the declarations of PB BR shooters (for many decades), most people still think seating is tuning, and that it's fine tuning. When someone with a hunting cartridge chooses not to be ITL, they pick seating as pulled out of their butts or book, and go into powder testing with it. Then, from a powder tune, they try seating testing. But of course changing seating from tune will collapse their tune at the same time as changing seating results in itself. Classic tail chasing.
It's all backwards, all wrong.

I'll argue this but for only two points.....In general, a hunter is obligated
by what the magazine restriction is. Second, and unlike A PB/BR shooter
who tunes for the match at hand, a hunter has to pick a happy medium
since his money shot will be an unknown distance using a totally different
bullet for the job.......I'll guarantee you that my primary wood chuck rifle
is loaded single shot and in the lands. I put a removable ramp in the mag,
to make life easier.

 

[jimmymac]

I didn't say I found best at 30thou OTL. I said a change of 30thou from my optimum.
I do very close to Berger recommended full seating testing. Coarse, and then finer (while away from any powder tune), and then finest within a seating window (group shaping after powder development).

This is how point blank BR shooters concluded that bullets had to be in the lands(ITL) to shoot well.
But that was rarely true outside of a tiny underbore.

The problem: PB BR shooters did not actually do correct full seating testing, as it was useless for their particular cartridges, which rely on a high starting pressure that seating ITL provides.
Their little wiggles while ITL amounted to no more than group shaping, and leaving land contact collapsed their tunes completely. So that left them thinking that seating was a fine tune from an incredibly limited place,, and that's true -with a 6PPC.
It's one of several areas where PB BR proves itself as non-applicable to the rest of the shooting world.

Now, thanks to the declarations of PB BR shooters (for many decades), most people still think seating is tuning, and that it's fine tuning. When someone with a hunting cartridge chooses not to be ITL, they pick seating as pulled out of their butts or book, and go into powder testing with it. Then, from a powder tune, they try seating testing. But of course changing seating from tune will collapse their tune at the same time as changing seating results in itself. Classic tail chasing.
It's all backwards, all wrong.

I’m not really sure what point you are trying to make. Tuning isn’t rocket science like many try to make it. Finding a good combination of seating depth and powder charge for a particular bullet is about all it takes from a hunting gun to a BR rig. I don’t care if I end up in the lands or jumping bullets. The targets will talk to me.

One of the first things I learned as a reloader trying to maximize the accuracy of hunting guns was that the ability to adjust seating depth was a game changer more so than powder charge changes. That’s my two cents.

 

[Wild Bill IV]

Everything is in the harmonic area

Well said!!! People don't realize that nodes can be destructive or constructive!! Those nodes are located along the horizontal axis and intersecting wave form and that horizontal axis when grafting the wave (frequency and amplitude)!!! I'm one of the old timers too before the age of internet!!!! I find a good charge, then work on seating depth, then fine tune with slight charge changes!!! Zeroing in on a constructive node!!!

 

[Wild Bill IV]

I'll argue this but for only two points.....In general, a hunter is obligated
by what the magazine restriction is. Second, and unlike A PB/BR shooter
who tunes for the match at hand, a hunter has to pick a happy medium
since his money shot will be an unknown distance using a totally different
bullet for the job.......I'll guarantee you that my primary wood chuck rifle
is loaded single shot and in the lands. I put a removable ramp in the mag,
to make life easier.

I'm a one shot big gamer!!! My loads don't fit in the mag!!! I shoot big game like you shoot chucks!!! One shot, through the heart and lungs!!! And hit them hard so they don't get up after rolling them!!!! Yeah!!! 160s in a 7-08 (2600f/s) at 250 to 600 yards!!! I like long impulse time!!! The longer that bullet spend time in the animal, the more reaction time for momentum, force, and energy transfer!!!

 

[mikecr]

Mike, why do you say that testing seating depth isn’t tuning ?

I say it because optimum bullet seating is independent of powder or barrel tuning.
The best primer/striking is independent of powder or barrel tune.
Fire forming to reach stable capacity is independent of tune.
They are matters prerequisite to tuning.

Tuning isn’t rocket science like many try to make it.

Yet there is so much about it that we don't fully understand, and can't predict.
I believe it would help all of us to acknowledge what is and is not 'tuning'.
It would help to consider what is coarse and fine in adjustment, and to understand that anything is calibrated coarse -before fine -before finest.

One of the first things I learned as a reloader trying to maximize the accuracy of hunting guns was that the ability to adjust seating depth was a game changer more so than powder charge changes.

This is true with hunting capacity cartridges. Seating is the coarse/biggest affect to results.
Powder is the finest, right to the kernel.
You can do seating testing any time, but I say it's best to search for best powder charge -with optimal seating in-place, and it's easier to find best seating while not collapsing a powder tune at the same time.

 

[abersfelderami]

You can do seating testing any time, but I say it's best to search for best powder charge -with optimal seating in-place, and it's easier to find best seating while not collapsing a powder tune at the same time.

 

[Wild Bill IV]

This graph is interesting, nothing wrong with these data as far as I can tell. I was not able to determine the significance of the open circle vs filled circle data points, due to a lack of patience, but if anyone sees that mentioned in the report, please reply.

If anyone reads the associated report just be aware that there are conclusions based on assuming Boyles Law for ideal gases. Combustion gases in a rifle chamber while pushing a bullet (mainly CO2 and water vapor) are at pressure conditions far exceeding ideal gas pressure. The critical pressure of CO2 is about 1071 psi, and for water it’s about 3204 psi. Both gases are what is known as supercritical under chamber pressure conditions, and by a wide margin. Any conclusions in the report assuming ideal gases and using Boyles law are far too simplistic and may not be reliable.

I read the report too!!! Smokeless propellant gases are not Ideal Gases!!! Like you stated, gaseous CO2 and H2O have different states due to pressure and heat!!! Steam has 8 know states due to changes in heat and pressure!!! In that report, there was no formulas that included Heat!!! From my research in Internal Ballistics, a knowledge of thermodynamics, hydrodynamics, and quantum mechanics are needed!!! Oh, don't forget rocket science since the internal shape of the case is elliptical and increases compression just like the 5 Saturn Rocket Engines have double elliptical housings that focus all the energy into a single piont!!

 

[Wild Bill IV]

Every gun I've worked with (probably not as many as most of you have worked with) has shown a sensitivity to bullet jump. In my limited experience there is a definite sweat spot that is .005"-.010" wide centered in the neighborhood of .050" (varies between gun/bullet combo). Like many of you my experience has convinced me that bullet jump matters and like many of you I've pondered why.

Since you don't know me I should tell you that I am a physicist/engineer/professional experimental scientist. The question of why bullet jump matters fascinates me to the point of obsession and is the singular reason I became interested in precision reloading. That fascination has driven me towards a new solution set that you'll find at superior-precision.com, by the way. I'm also a machinist/mechanic with a lifetime of real world hands-on experience working with metals, etc. So, as for why bullet jump matters, I have spent a lot of time thinking about this. As a professional experimental scientist I've also spent a lot of time pondering how to explore this in a way that would provide clear answers to the question but that would require resources beyond my means. Not to be discouraged I've been left with whatever this fatty blob between my ears can imagine. Here are my thoughts....

I suspect that it's all about the velocity of the bullet when it engages the lands. As the bullet engages the lands it is deformed, some say engraved. Some of you are machinists. You know that cutting speed matters, you know that the surface finish you get depends on surface feet per minute (velocity of the material at the cutting edge). If you go too slow and the surface looks like crap as the material tears at the cutting edge. Increase the speed and you find a point where the surface finish improves because the material is now fracturing at the cutting edge instead of tearing. Go too fast and you burn up the cutter and the material. I suspect that the same thing is happening here but the result will depend upon the bullet (jacket design, etc) and the rifling (5R vs square, etc) and I suspect that some combos are less sensitive that others. I suspect that how the jacket deforms is very dependent upon the velocity; if too slow it deforms unevenly causing the bullet to drift off center, if too fast then other things start to happen.

When the primer explodes (unlike the powder which burns), the resulting very rapid rise in pressure is more that enough to push the bullet into the lands which probably happens before the powder ignites. That pressure is accelerating the bullet. Start with the simplifying assumption of constant pressure during this time period therefore constant acceleration. Per classic equations of motion V^2=2ax. Double bullet jump (x) and V increases by a factor of sq.rt of 2 which is 1.4x. Go from .005 to .050 and you've changed jump by a factor of 10 so the velocity of the bullet when it hits the lands increases by a factor of sq.rt of 10 = 3.16x. The simplifying assumption is probably incorrect so the actual increase in velocity if probably much higher. Anyway...the important part of the thesis is that if the bullet hits the lands too slowly the jacket material tears/deforms in an irregular way. Go fast enough and the deformation (engraving) is more consistent.

However, increasing bullet velocity as it enters the rifling also increases the angular acceleration as the bullet goes from zero rpm to whatever it needs to be to move forward. Double the velocity and you've doubled the angular velocity that the bullet needs to suddenly achieve which doubles the forces between the rifling and the jacket. At some point the jacket material will not support that much rotational acceleration and it will start to tear.

So, I surmise (without any direct evidence to support it aside from a lifetime of working with metals) that if the velocity of the bullet when it hits the lands is too small the jacket undergoes non-uniform plastic deformation which tends to result in the bullet becoming non-concentric and if it's too high the jacket undergoes non-uniform brittle deformation which tends to result in a non-uniform jacket surface when it leaves the barrel. Both cause irregular flight. Somewhere in the middle there is a sweet spot. Data to test the thesis would require some fairly expensive equipment which I'd love to play with but don't have so I stay focused on the pragmatic reality that why bullet jump matters doesn't matter. I do the load development work req'd to find the sweet spot then rely on my seater die to consistently give me the bullet jump I need.

I have a BS in Industrial Physics with a AS in Mathematics!! My thesis is based on Harmonics!! Frequency, amplitude, constructive and destructive nodes!! There can be one, maybe two, constructive node(s) in seating depth!! I have found that secant ogive bullets with less than 12 or 13R love touch to 0.005 jump!! Greater than 12 or 13R, is a guessing game!! Seems that long bearing surface bullets like that depth!!

The constructive node occurs when the bullet exits the barrel whipping upward and inline with the true barrel axis!!! Just like jumping off a balanced diving board!!! The destructive node occurs on the downward whip axis alignment!!! You don't go anywhere but down and out off that diving board node!!! Bullet acceleration (changing velocity) is the key to finding nodes!!! And the proper pressure curve is the function of that acceleration!!! A little change in seating depth changes the gas density slightly in the chamber before the bullet can break the static (stationary) friction! Seating depth fine tunes the pressure curve!!! To achieve precision, concentrate on fine tuning acceleration which is proportional to the square of the changing velocity!!!

 

[jimmymac]

I say it because optimum bullet seating is independent of powder or barrel tuning.
The best primer/striking is independent of powder or barrel tune.
Fire forming to reach stable capacity is independent of tune.
They are matters prerequisite to tuning.

Yet there is so much about it that we don't fully understand, and can't predict.
I believe it would help all of us to acknowledge what is and is not 'tuning'.
It would help to consider what is coarse and fine in adjustment, and to understand that anything is calibrated coarse -before fine -before finest.

This is true with hunting capacity cartridges. Seating is the coarse/biggest affect to results.
Powder is the finest, right to the kernel.
You can do seating testing any time, but I say it's best to search for best powder charge -with optimal seating in-place, and it's easier to find best seating while not collapsing a powder tune at the same time.

I am a simple guy. This does not have to be made difficult. Tuning is pretty simple. If you are in to knowing the whys then have at it. I’m more interested in just getting to a state of tune in as few shots as possible for my stated purposes. I can generally do this in 50 shots or less.

In regard to powder charge, I do not need my load to be accurate to the kernel. I’ve found that there is a cushion on either side of my optimal powder charge that will give me acceptable results for the intended application. Additionally, most people don’t have the equipment and the skills to shoot the difference anyway

This will be my last post on this topic. Tuning loads and making high quality ammo that performs to expectations isn’t difficult. I have no desire to end up in the weeds obsessing over minutia. My targets tell me I’m okay. YMMV

 

[mikecr]

the guys that I follow consider every aspect of optimizing components part of the tuning process, I have know idea why we need to re - establish terminology.

Brass prepping is 'optimizing components'.
But it is prerequisite to tuning. Not tuning in itself.

Tuning amounts to adjustment of:
-powder
-barrel
-system
Each, independent of the others.
However, most of us stop on powder that hits on barrel tune, while assuming our system is -not broken.
This leaves us with best grouping at a fixed range, with horrible SD, and if the grouping isn't good enough, most are not considering the possibility that the gun doesn't like the resting/balance/or hold.
All detrimental for hunting accuracy.

Now add abstracts like changes to seating, primers, or neck tension, and you've gone from tuning to tail chasing. All this should have been worked out beforehand, per an actual plan.

Ideally, all prerequisite testing would be completed before moving to ACTUAL tuning.
You would dial in powder across a good chronograph for tightest ES/SD.
You would then dial in your barrel tuner for tightest grouping.
Then for hunting, you would move to cold bore accuracy testing.
All testing with intended field rest and conditions, and some experimenting here & there.

Most of us don't use tuners, and our capabilities are then limited to a barrel tune while out of powder tune. This works for precision at fixed/tested ranges,, not so much for hunting accuracy. So hunting accuracy expectations are often lowered. That's ok.
Both are ok, but we could understand it. Maybe work around it.

 

[Fuj]

Brass prepping is 'optimizing components'.
But it is prerequisite to tuning. Not tuning in itself.

Ahh, but you may want to tweek on your neck tensions after
the fact, avoiding the word, tune.