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Do I need to neck turn?

 
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  #22  
Old 06-20-2013, 07:16 AM
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Re: Do I need to neck turn?

Quote:
Originally Posted by Mikecr View Post
Bart, the problem with a 'bullet release force' measurement is that it does not actually represent 'bullet grip' , or tension. In fact, if not controlled, it might mean nothing at all.
Mike, I’m thinking static rather than dynamic conditions. And I’m also referring to the force needed to push the bullet out of the case when it’s fired, not the force needed to seat the bullet in the case measured with with any electronic or mechanical tool as you mention. To say nothing of the surface characteristics of the case mouth and bullet jacket which will cause more or less resistance to moving the bullet for a given compression force (neck tension?) the neck walls put on the bullet. Nor is the issue with cases with unturned necks with thicker neck walls at the shoulder and thinner ones at the mouth which is often the way things are. Besides, the ammo making industry typically uses “release force” as the physical stuff needed and it’s what most folks easily relate to and can easily measure themselves with inexpensive tools.

Info from another forum on this subject.......

A fellow who had participated in the HP White tests in the 1970's said they'd found bullets didn't start moving until about 10,000 psi. QuickLOAD thinks it's more like 3600 psi. Force on the bullet's equal to pressure in psi times cross sectional area of the bullet. Pressure increase partly stalls as the volume expands as the bullet slips from case neck to throat. There's at least one plot in the 1965 Lloyd Brownell study (link below) showing pressure flats at around 12,000 psi. So this can vary.

http://deepblue.lib.umich.edu/bitstr...3.0001.001.pdf

In the dynamic system, even though bullet pull is only 60 lbs slipping from the neck under the kinetic friction coefficient, given the short length of time the bullet has to start moving, its inertial resistance is a significant addition to that. The result is that instead of graciously popping forward, the neck is actually expanded from the rear by pressure, rolling forward almost to the case mouth before the bullet is fully released. This is the reason the mouths of fired cases are usually curled inward a little bit as compared to the rest of the neck. The rest of the neck expanded, but as the expansion neared the mouth, gas started leaking past it and equalizing pressure on the other side, so the mouth ceased expanding. We know this begins before the bullet has obturated the bore because super high speed photos show gas and powder particles preceding the bullet at the muzzle. Dr. Brownell attributed the effect of seating distance off the throat to the amount of greater gas bypass the deeper seating allowed.

Once you get to the rifling, Harold Vaughn showed 6mm (or .270) need around 1200 lbs of force to swage them into the rifling. That was about 20,000 psi static pressure equivalent, but assuming the kinetic coefficient of friction is about half the static coefficient of friction, about 10,000 psi would be right under dynamic conditions and with the bullet already moving.
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  #23  
Old 06-20-2013, 11:18 AM
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Re: Do I need to neck turn?

I'm pretty sure the neck has completely released before there is any bullet movement, and that 'pull force' means nothing really.

-Take 10 cases with same neck tension, and polish/alcohol wash neck IDs on 5. Load all the same, uncoated/cleaned bullets, & shoot across a chrono. You will see no difference.
This, even though seating/pull forces are significantly different with the squeaky clean necks, and challenging our ability to seat bullets equally with uncleaned necks.
-Now test 10 cases with 5 partial neck sized 1/4 length, and 5 partial sized 3/4 length. This is where you will see a difference across a chronograph.

It is simple grip force that affects bullet release, and not friction.
Yes, there is blow by, before the bullet even moves, AND before the necks have opened fully to seal.
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  #24  
Old 06-20-2013, 02:02 PM
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Re: Do I need to neck turn?

Mike, please explain how case necks enlarge diameter off bullets without moving the bullet with all that pressure on the back of the bullet. Does the neck start getting larger than bullet diameter starting at the mouth, or at the neck-shoulder junction?

How any bullet could remain in place after 99% of the case neck's expanded off of it and only a tiny smidgen of mouth area holds the bullet in place before all bullet-neck contact is gone and the bullet then moves. . . .

Last edited by Bart B; 06-20-2013 at 03:42 PM.
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  #25  
Old 06-20-2013, 05:04 PM
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Re: Do I need to neck turn?

Expand the neck .000001 and the bullet is completely free and gasses are passing it. Not much pressure needed to do that, which I imagine builds at the nearest bullet neck junction and moving forward around the bullet.
Until then, a bullet rests with it's mass gripped by that neck, and isn't going anywhere fast.
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  #26  
Old 06-20-2013, 05:24 PM
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Re: Do I need to neck turn?

Quote:
Originally Posted by Mikecr View Post
Expand the neck .000001 and the bullet is completely free and gasses are passing it. Not much pressure needed to do that, which I imagine builds at the nearest bullet neck junction and moving forward around the bullet.
Until then, a bullet rests with it's mass gripped by that neck, and isn't going anywhere fast.
I'm assuming you're referring to a dimension of one millionth of an inch (.000001").
If that's what you meant, no one can mechanically measure a dimension that small. Even precision gauge blocks aren't held to a tolerance that small.
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  #27  
Old 06-20-2013, 05:48 PM
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Re: Do I need to neck turn?

Quote:
Originally Posted by Mikecr View Post
Expand the neck .000001 and the bullet is completely free and gasses are passing it.
What about bullet and case surfaces with a 6 microinch finish? The case neck will have to expand more than 1 microinch to ensure all the micro-mountain tops clear those other ones on each surface. Meanwhile, some micro-mountain tops are down in micro-valleys and friction will be at hand when they move against each other.

No case neck insides nor bullet jacket has surface smoothness that small.

C'mon Mike. That's rediculous.
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  #28  
Old 06-20-2013, 06:05 PM
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Re: Do I need to neck turn?

Quote:
Originally Posted by Bart B View Post
What about bullet and case surfaces with a 6 microinch finish? The case neck will have to expand more than 1 microinch to ensure all the micro-mountain tops clear those other ones on each surface. Meanwhile, some micro-mountain tops are down in micro-valleys and friction will be at hand when they move against each other.

No case neck insides nor bullet jacket has surface smoothness that small.

C'mon Mike. That's rediculous.
You're completely correct. Not only that, but the diameter of a bullet isn't held to that tolerance.
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