Tension Testing - It is official!!! According to Instron there is such a thing as tension testing

[Bart B.]

Well, Bart you can go ahead and do this 100 more shots to believe it..

I did it with a dozen shots with about 3-pound extraction force and 12 more with about 60 pounds. Fired alternately to even out effects of fouling the bore. 3-pound ones had different velocity average than the 60's. About 30 to 40 fps slower and less extreme spread.

Someone took the expander ball out of his 30-06 full length sizing die then stuffed his component recipe in cases full length sized in it. He said he wanted to try sizing cases in a die like I used. But he forgot about me telling him to have the die neck honed out to a couple thousandths less than his earlier reloaded ammo. The primers were mushroomed out past primer pocket radiused edges and dented less than his other normal looking. I asked if he had any more of that ammo bud he said he pulled the bullets from them. And it took a lot more force on his press handle to pull those cases down off the bullets than normal.

At a rifle match years ago, one shooter was complaining about his ammo having elevation stringing. And his 1000 yard zero was less elevation on his rear sight than normal. After his rifle smoked and recoiled a lot more than normal he quit shooting after he noticed the ejected 30-06 case had no neck. It went down range with the bullet. Pulling some of the bullets showed the had different levels of dissimilar metal bonding corrosion "glueing" the necks in place and bullets having much higher than even MIL SPEC 60 pound release force. The ammo was many years old and he wanted to shoot it.

 

[dcali]

What on earth...

 

[Ron Blain]

Yes it could be done but not practical. We don't own tensile testers. The tester pulls at a constant rate (distance of movement). The plot would be pounds force vs time or extension. I always believed if you cannot see the difference on the target it doesn't matter. The results would probably open a big can of worms.

That has been pretty much my thinking. We have a cartridge which inside about 1.5 mSec the peak pressure is over and the bullet is on its way. Many cartridges developing upward of 60,000 PSI of chamber pressure so how much does bullet hold or neck tension really matter? So while it can be interesting stuff you have to wonder what the overall effect really is?

Ron

 

[mikecr]

One of the three principle ways that we tune is with with bushing size, all sizing the same amount of the neck. It shows up on the paper which trumps the chronograph. I think that it may be because of the time to muzzle issue.

Much of this would have to be qualified to make any sense of it.
Not everybody uses bushings. We don't all shoot the same cartridges, and your declaration does not work for all bushing users.
So there is either rational and logical reasons for your notion here,, or not.

 

[BoydAllen]

Much of this would have to be qualified to make any sense of it.
Not everybody uses bushings. We don't all shoot the same cartridges, and your declaration does not work for all bushing users.
So there is either rational and logical reasons for your notion here,, or not.

Notion! I have done what I posted about...you know (I think) actual testing. Damn near everyone in the short range benchrest game uses bushing dies. Most that I have known have tried different ID bushings until they found the one that gave the best accuracy. The common Harrell dies are not set up to partial size necks. I find your comment intentionally rude, and dismissive.

 

[dkhunt14]

That has been pretty much my thinking. We have a cartridge which inside about 1.5 mSec the peak pressure is over and the bullet is on its way. Many cartridges developing upward of 60,000 PSI of chamber pressure so how much does bullet hold or neck tension really matter? So while it can be interesting stuff you have to wonder what the overall effect really is?

Ron

Might be so. It short range and long range BR guys use bushing size to tune. I have seen it make a big difference on a thousand yard target. Matt

 

[BoydAllen]

There are basically three tuning variables for a given powder, bullet, primer, and barrel; charge weight, bullet seating depth, and neck tension. In the case of the latter, although the common custom is to express it in terms of the amount that the size neck is expanded by the bullet being seated, the actual effect depends on the thickness of the neck, how hard it is (brand of brass, work hardening, annealing) and of course the ID of the bushing or neck portion of the die. In the short range game I am not aware of much being done in the way of tuning by varying the amount of the neck that is sized. This is probably because there is not a lot of bullet shank in the neck due to the length of the bullets, and the well established fact that one of the most popular powders "likes" about all of the neck tension that can be had, especially when the chamber requires neck thickness in the range of .008-.0086. There is also the matter of making sure that the pressure rings of seated bullets do not go past the sized part of the neck, something that is generally viewed as undesirable.

 

[SheepDog]

There is one thing about tension and bullets that we should all try to remember. The bullet is not being pulled out of the case. Pressure is pushing the bullet out and at the same time it is expanding the case. The bullet is under compressive forces and not tensile forces at the time of use. The pressure/time rise that occurs at firing is releasing the grip between the case and bullet at the same time it is pushing the bullet down the bore. If you see carbon on the neck of the brass then you know the bullet left before the case expanded enough to seal against the chamber. If there is no carbon on the neck then the case had expanded to the chamber before the bullet exited the case. At no point in this process does the physical quantity of tension apply to the movement between the bullet and case.

 

[Ron Blain]

There is one thing about tension and bullets that we should all try to remember. The bullet is not being pulled out of the case. Pressure is pushing the bullet out and at the same time it is expanding the case. The bullet is under compressive forces and not tensile forces at the time of use.

Matter of fact..... My only goal throughout all of this is uniformity.

Ron

 

[dkhunt14]

There is one thing about tension and bullets that we should all try to remember. The bullet is not being pulled out of the case. Pressure is pushing the bullet out and at the same time it is expanding the case. The bullet is under compressive forces and not tensile forces at the time of use. The pressure/time rise that occurs at firing is releasing the grip between the case and bullet at the same time it is pushing the bullet down the bore. If you see carbon on the neck of the brass then you know the bullet left before the case expanded enough to seal against the chamber. If there is no carbon on the neck then the case had expanded to the chamber before the bullet exited the case. At no point in this process does the physical quantity of tension apply to the movement between the bullet and case.

All the carbon on the neck or shoulder tells you is how hard or soft your brass is. I believe the bullet starts moving before the case expands. A primer and no powder will push a bullet a ways down a barrel without expanding a case. Before pressure can build I believe the bullet has to seal the bore. Just my thoughts. If not, tension wouldnt effect velocity or group and it does. Matt

 

[BoydAllen]

There is one thing about tension and bullets that we should all try to remember. The bullet is not being pulled out of the case. Pressure is pushing the bullet out and at the same time it is expanding the case. The bullet is under compressive forces and not tensile forces at the time of use. The pressure/time rise that occurs at firing is releasing the grip between the case and bullet at the same time it is pushing the bullet down the bore. If you see carbon on the neck of the brass then you know the bullet left before the case expanded enough to seal against the chamber. If there is no carbon on the neck then the case had expanded to the chamber before the bullet exited the case. At no point in this process does the physical quantity of tension apply to the movement between the bullet and case.

It does not matter. All that we need to know is that "neck tension" affects tune, and that lack of uniformity degrades accuracy. About all that we reasonably have available to monitor this situation as we load ammunition are force gauges that rely on a stack of bellville washers or hydraulic pressure. We commonly ignore factors that effect the amount of force that will be required to push a bullet from a case neck by referring to neck tension as the difference of the diameters of sized and loaded case necks, probably because so few of us have a way to measure the force required to seat bullets. In any case we know from testing that differences in how hard case necks grip bullets affect tune.

 

[mikecr]

Boyd, your further posts at least partially qualify the declaration.
That qualification being bushing neck sizing length beyond seated bullet bearing.

Underbores like pressure. That's fast powders and high starting pressure. So they like mega bullet grip for tension and/or jam land relationship.
By sizing an amount greater than spring back -for greater length than seated bearing, you cause binding action against the base/bearing junction. That is, a seating bullet is left in max upsizing force right at the base. This does provide more tension, and the harder you put the base into bind, the greater the bullet grip.

This is not common in LR, with LR bullets, in LR capacity cartridges.
It would be very undesirable for LR cartridges(which don't rely on mega bullet grip), as their seated bearing is often deeper & more near donut area. With unturned necks and/or donut area brought into tension, via sizing length beyond bearing, or worse, FL neck sizing, tension variance goes through the roof.
Tension variance matters with LR cartridges, typically running slower powders and 65Kpsi max viable loads.
Not so much with a tiny 6PPC running ~75Kpsi or more(competitively).

I likely stated earlier that tension is merely spring back against seated bearing.
And that tension is adjusted with length/area of that spring back against bearing. These are true, but do not integrate qualifiers, like thickness, hardness, base binding, and even shoulder angles(for FL NS). Maybe I should have tried to sum all that up another way.
I did not mean to offend. Just desire to keep the discussion clear and logically useful.

 

[mikecr]

All the carbon on the neck or shoulder tells you is how hard or soft your brass is. I believe the bullet starts moving before the case expands. A primer and no powder will push a bullet a ways down a barrel without expanding a case. Before pressure can build I believe the bullet has to seal the bore. Just my thoughts. If not, tension wouldnt effect velocity or group and it does. Matt

If this were true, that bullets had to overcome pull forces, many guns would blow up with current loads. It would also mean you could vary MV with no more than a friction change to bullet pull(same tension). That doesn't happen either.
Aside from reduced pressure loads, neck sooting is often due to excess chamber end clearances and high neck clearances.

 

[Lapua40X]

... The bullet is not being pulled out of the case. Pressure is pushing the bullet out and at the same time it is expanding the case. ... At no point in this process does the physical quantity of tension apply to the movement between the bullet and case.

I really wanted to stay out of this but I feel an obligation to raise one minor point with respect to your statement here. Where pressure is "pushing" the bullet out and at the same time expanding the case, then there has to be some amount of drag on the bullet during this cycle. The cycle is not 1. neck expands 2. bullet moves forward. It is 1. neck expands as bullet moves forward. So, IMO, the factor of release force, however slight, must be included in the equation. Therefore, I disagree with your point that "At no point in this process does the physical quantity of tension apply to the movement between the bullet and case."

 

[mikecr]

I believe the necks expand the ~0.000,000,000,001" needed to fully release a bullet, before a bullet moves one planck length.
That's nowhere near fully expanded but plenty enough, and takes very little actual pressure to cause it.
On the other hand, it could take way more pressure to overcome neck friction.

Consider the guy that's just SS pin cleaned necks to metal, way oversized necks, and squeaks clean bullets into them. If his powder burn were to hit that friction(like a wall) the gun would likely be damaged right there. We would all know about this.

 

[BoydAllen]

Boyd, your further posts at least partially qualify the declaration.
That qualification being bushing neck sizing length beyond seated bullet bearing.

Underbores like pressure. That's fast powders and high starting pressure. So they like mega bullet grip for tension and/or jam land relationship.
By sizing an amount greater than spring back -for greater length than seated bearing, you cause binding action against the base/bearing junction. That is, a seating bullet is left in max upsizing force right at the base. This does provide more tension, and the harder you put the base into bind, the greater the bullet grip.

This is not common in LR, with LR bullets, in LR capacity cartridges.
It would be very undesirable for LR cartridges(which don't rely on mega bullet grip), as their seated bearing is often deeper & more near donut area. With unturned necks and/or donut area brought into tension, via sizing length beyond bearing, or worse, FL neck sizing, tension variance goes through the roof.
Tension variance matters with LR cartridges, typically running slower powders and 65Kpsi max viable loads.
Not so much with a tiny 6PPC running ~75Kpsi or more(competitively).

I likely stated earlier that tension is merely spring back against seated bearing.
And that tension is adjusted with length/area of that spring back against bearing. These are true, but do not integrate qualifiers, like thickness, hardness, base binding, and even shoulder angles(for FL NS). Maybe I should have tried to sum all that up another way.
I did not mean to offend. Just desire to keep the discussion clear and logically useful.

 

[BoydAllen]

Good post, but I would like to politely point out a couple of things. I know of a very successful 1,000 yard shooter that uses a powder that, based on his testing, likes more resistance to bullet movement (How's that ?) than he can get with annealed cases, so he does not anneal for that powder for that caliber.

On the PPC side, not all powders crave smaller bushings. Some time back I was helping a fellow on the opposite coast get started with his first PPC. He was shooting LT32 and during our discussion I discovered that he was using a bushing size that while appropriate for 133 was at least .001 too tight for his powder. He made the change and his groups improved.

On the other hand, I have thought that because of sectional density of long projectiles and the burn characteristics of slower powders that a smaller percentage of the back pressure against the burning powder is attributable to bullet pull, and that in those situations reactions to increases in bullet pull might be less precipitous.

 

[SheepDog]

It would be an interesting test to use a controlled air pressure to see how much pressure it takes to push the bullet out of the case. Remember that in most cases this would involve the engraving of the bullet into the lands at least part way. The bullet is typically still in the case until the engraving process begins. At 3000 psi there is only 223.5 pounds of force on a 30 caliber bullet. Would that be enough to initiate the engraving process?

Lapua40x,
At no time in the process is anything pulling the bullet and the case apart. Tension is a pulling force. Since nothing is being pulled I stand by my statement.

 

[mikecr]

I know of a very successful 1,000 yard shooter that uses a powder that, based on his testing, likes more resistance to bullet movement (How's that ?) than he can get with annealed cases, so he does not anneal for that powder for that caliber.

Sounds right to me. Harder cases (than freshly annealed) provide more spring back force against bearing.
On the flipside, freshly process annealed, if done very well, provides more consistent spring back force(albeit low).
Either way I imagine load development could benefit from tension testing, and ultimately needs to be finished with tension as it will be forever managed.

 

[Bart B.]

For a given case neck grip on a given bullet, I don't think it matters if your push the bullet out from the back with a pin through a hole in the case head or pull the bullet out with a bullet puller. The same force to move the bullet will be needed.

Mount a shell holder upside down above a hole in something clamped to bench top so it's past the edge. Slide a case in it then put a collet type bullet puller tight on the bullet. Affix a tin can to the bullet puller then slowly but incrementally load it with bullets. When the bullet gets pulled out of the case, weigh everything whose weight did that.

One could use a hand scale affixed to the bullet puller and get numbers good enough to make comparisons