Seating depth theory

[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!!!

 

[JFrank]

believe it would help all of us to acknowledge what is and is not 'tuning'.

Not speaking for another man but 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.
They can speak for themselves of course.

 

[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.

 

[JFrank]

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

I could agree with you on brass prep but certainly have to add seating adjustments, primer testing per action and barrel as well as bullet hold to the list of tuning procedures. I’m not alone in this line of thinking.
The good part is we only have to please ourselves.

Thanks for the reply Mike , always good to chat with you.
Jim

 

[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.