checking neck tension

[stebbins243]

Is there a way to check neck tension? I know some presses have pressure pads and other ways to measure as you seat the bullet. But i was wondering on unfired brass.

 

[nonliberal]

Measure the neck, seat a bullet, then measure the neck again. The difference is your neck tension.
A little less accurate way if you cant seat a bullet would be to measure the inside diameter, the difference between that measurement and the caliber your using is the neck tension.

example- inside diameter is .304, your using a .308 caliber bullet, your tension is .004

 

[BoydAllen]

Although we commonly use the difference between loaded and unloaded neck diameters as a way to describe "neck tension", I think that what is more important is the amount of force that it takes to move a bullet that has been seated, which for various reasons can vary, even though the difference in diameters may be the same. Making a direct measurement of the force that it takes to move a bullet in the direction that it will move when fired can be done, but it in not convenient, and does not lend itself to checking every round as it is loaded. If that is what is wanted, a good method is to use an arbor press type of seater, such as Wilson makes, in conjunction with an arbor press that has a built-in force measuring device, such as K&M offers. You might want to read the article that this URL leads to.
http://bulletin.accurateshooter.com/2012/02/the-complexities-of-neck-tension-why-bushing-size-is-only-one-factor-to-consider/

 

[mikecr]

There is currently no tool available to directly measure neck tension.

I don't agree that seating force, or pulling force, correlate directly with neck tension. Nor do I believe these forces amount to any ballistic significance, like actual neck tension does.
For basis: Anyone can dramatically increase seating forces with over annealing, while neck tension is actually REDUCED with this.

 

[Alex Wheeler]

Question is, does the bullet get pushed out of the case before the neck expands or does it happen simultaneously? No real way of knowing. If its pushed out the k&m measurement should be helpful, if the neck is expanding at the same time, the neck "stretch" comes into play.

 

[BoydAllen]

Generally, we know that uniformity of bullet seating force helps with lowering the ES of velocities. This tells us that there is some effect on how pressure builds in the case. Beyond that, I have done experiments using different diameters of neck bushings, and the targets made it clear that the rifle had definite preferences. We may not be able to directly observe what is taking place, but we can see the results.

 

[jlow]

I have a question for the OP, you mention “pressure pad” to measure bullet seating, can you expand on that a bit?

 

[stebbins243]

Pressure pad or load cell, they can either be mechanical or electric. On an arbor press the die would sit on top of it. As you press down it will messure the force pushing down to seat the bullet.

 

[243winxb]

The military uses • BULLET PULL: Minimum individual 35 lbs. for 5.56 ammo. To test mine, i used a bath room scale & leverage to push the bullets into the case of dummy rounds. When the brass was expanded by .002" on bullet seating, i measured 35 to 45 lbs to push the bullet into the case. Not precise enough for benchrest, i know. ;D Edit/add- Not testing during seating of the bullet. This is after seating. I wanted to see how much pressure was needed to start the bullet moving again. The pressure may be different between seating & starting the bullets move inward again.

 

[jlow]

Thanks, yes I understand the concept, the question is what specific setup have people been using and does it work? Where can a person get one? I can see that as per 243winxb’s comment, a bathroom scale may work as a test, but ideally one could incorporate it into regular loading as a QA control?

One concern I would have is that the pressure pad/load cell would have some give and so your seating depths might change but I guess it and the added height of the pad/load cells can be adjusted for.

 

[bozo699]

As usual Boyd's post's make good sense and Mikecr has a valid point as well, basis as he says; you take two cases both with the same firings, both sized with a bushing to give .001 neck tension, 1st case leave the inside case neck untouched and the second case polish the inside of the neck with 0000 steel wool and see which one takes more seating force! both has .001 neck tension but on a K&M arbor press w/force seat system the first one may take say 30# to seat and the second one say 90# to seat so as Mike points out neck tension and seating force are two different things!
Wayne.

 

[Minesweeper3433]

Thanks, yes I understand the concept, the question is what specific setup have people been using and does it work? Where can a person get one? I can see that as per 243winxb’s comment, a bathroom scale may work as a test, but ideally one could incorporate it into regular loading as a QA control?

One concern I would have is that the pressure pad/load cell would have some give and so your seating depths might change but I guess it and the added height of the pad/load cells can be adjusted for.

When using an arbor press your seating depth would not change as all you are doing is putting another flat object beneath your dies. And as the dies goes off of the adjustments made to the die all that would change is how far you must rotate the handle to fully seat. Your ram movement would decrease the width of the pressure pad. This is presuming however that you are using a solid type pad with parallel plates and not a soft pad.

 

[jlow]

Thanks, yes I understand the concept, the question is what specific setup have people been using and does it work? Where can a person get one? I can see that as per 243winxb’s comment, a bathroom scale may work as a test, but ideally one could incorporate it into regular loading as a QA control?

One concern I would have is that the pressure pad/load cell would have some give and so your seating depths might change but I guess it and the added height of the pad/load cells can be adjusted for.

When using an arbor press your seating depth would not change as all you are doing is putting another flat object beneath your dies. And as the dies goes off of the adjustments made to the die all that would change is how far you must rotate the handle to fully seat. Your ram movement would decrease the width of the pressure pad. This is presuming however that you are using a solid type pad with parallel plates and not a soft pad.

Minesweeper3433 – in theory, I would agree with you, however, what I don’t know is if there is more deflection on the pressure pad/load cell as the force increases or if it remains constant. Only the latter situation would certainly give you constant seating depth. This is the reason why I was interested in finding out if someone use this method, what they use, and what their experience was?

 

[mikecr]

RSI LoadForce.
An electronic version of K&M's mechanical approach.
Sell you one if you're interested..

A neck only needs to expand below our measure(0.0001) and the bullet is totally free.
A sneeze would cause this tiny opening, which is far less than seating force friction. But it's enough still to affect peak pressure and barrel time.
I'm pretty sure someone, maybe Vaughn, determined that complete bullet release occurs before any forward movement.
I don't know how it was proven, but it passes tests.

IMO, the only time seating force variances represent a valid and direct correlation to tension, is when all necks are carefully controlled to present the same apples to apples measure.
This is easy enough to do, but only when you understand that seating force and tension are completely different animals.

 

[jlow]

Thanks mikecr – that is an interesting instrument indeed and with a price to match! Not in my budget right now but you never know…

Would like to explore this relationship between seating force and neck tension a bit more if I may. The way I understand this (and inputs would be much appreciated for my learning…), straightly speaking neck tension by definition is how much force is on the bullet after it is seated. Parameters which may affect this would include:

1) Differential OD of the bullet and in ID of the neck opening.
2) Consistency of the shape of the neck opening this would include donuts but not limited to this problem.
3) Consistency of the shape of the bullet.
4) Consistency of the bullet seating depth.
5) Neck thickness and its consistency.
6) Degree and consistency of the spring back of the neck brass i.e. as per annealing or lack of.

Seating force would be affected by all of the above plus:

7) Smoothness of the surface of the bullet.
8 ) Smoothness of the surface of the inside of the neck.
9) Existence of any lubricant on either the bullet or inside of the neck or both.
10) Concentricity of the bullet seat.

So going back to your comment “the only time seating force variances represent a valid and direct correlation to tension, is when all necks are carefully controlled”, would items 7-10 be it?

I would be interested in how one would prove that the “complete bullet release occurs before any forward movement”, perhaps Vaughn can elaborate?

The other question here is if this was 100% true then how does items 1, 2, 5, 6, 8 affect the final MV of the bullet? I mean if the bullet has been completely released before it moves, those parameters should have absolutely no affect on MV unless it has distorted the bullet?

 

[Minesweeper3433]

Thanks, yes I understand the concept, the question is what specific setup have people been using and does it work? Where can a person get one? I can see that as per 243winxb’s comment, a bathroom scale may work as a test, but ideally one could incorporate it into regular loading as a QA control?

One concern I would have is that the pressure pad/load cell would have some give and so your seating depths might change but I guess it and the added height of the pad/load cells can be adjusted for.

When using an arbor press your seating depth would not change as all you are doing is putting another flat object beneath your dies. And as the dies goes off of the adjustments made to the die all that would change is how far you must rotate the handle to fully seat. Your ram movement would decrease the width of the pressure pad. This is presuming however that you are using a solid type pad with parallel plates and not a soft pad.

Minesweeper3433 – in theory, I would agree with you, however, what I don’t know is if there is more deflection on the pressure pad/load cell as the force increases or if it remains constant. Only the latter situation would certainly give you constant seating depth. This is the reason why I was interested in finding out if someone use this method, what they use, and what their experience was?

What if you adapted a die set to have a cut off switch that would signal peak reading at say .001from fully seated so that it stopped reading once the dies was within a thousand of bottoming out?

 

[BoydAllen]

Harold can not elaborate because he as gone to that great rifle range in the sky. I looked through my copy of his book, and did not find any reference to a case neck releasing the bullet before it moved. Perhaps I overlooked something, and someone who has a copy can correct me? As far as his conclusion about the effect of seating force on velocity and accuracy goes, I think that it is entirely possible that the rifle he used was not a good enough tool for the results to be taken seriously. Although short range benchrest shooters do not have much use for chronographs, they do pay a lot of attention to accuracy, and the rifles that they draw their conclusions from are a lot more accurate than the one that he used for his test. They can see differences in accuracy based on what bushing case necks are sized with, and seating into the lands does not negate this. Harold did a lot of important and original work, but I think that it is important to pay close attention to all the details of a particular experiment before concluding that his results are useful. If you have read his book, I would refer you to his comments on O'Connor bedding as an example. or the fact that a lot of his work was done with a 721 Remington chambered in either .270 or 6mm Remington. Nevertheless he did some very original things that have been very thought provoking.