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Neck Tension
Discussion regarding the reloading of ammunition and tuning of loads for accuracy
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slimjim
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PostPosted: Wed Sep 12, 2012 3:05 am    Post subject: Neck Tension Reply with quote

I recently changed my dies and upgraded (I thought) to a single stage press which resulted in my favorite loads not being nearly as accurate as before.

www.huntingnut.com/ind...pic&t=9417

In the process of trying to figure out all my issues, I've pulled a lot of .270 bullets - lots of bullets (thankfully I have a Hornady Collet Bullet Puller). I have found a significant variation in neck tension with cases run though the same resizing die. By significant, I mean some bullets come out like butter while others I don't have the strength to extract them and must slam the handle multiple times to get them out.

In the interest of discussion, I'm posting what I've found out so far.

First, I was using a Redding S-type .270 Win FL resizing die with a 0.300 button. With new, unfired brass that was run through this die, bullets would insert and extract like butter. After one firing (and no cleaning), I could feel increased pressure during bullet seating. The light bulb came on when I had to extract some of these reloads and they were as difficult as any I had done before. Note, I have found insertion force to always be significantly more than extraction force.

I had thought the variation in extraction forces might be due to work hardening of the brass after multiple reloads. These once fired cases proved that it was not. My theory is once a case is fired, the inside of the neck is no longer shiny smooth and the brass becomes pitted which increases the coefficient of friction. I also think the carbon coating increases friction also. Thus, neck tension is least in new brass and greatest in fired brass with carbon residue inside. I also suspect that fired brass that is clean of carbon inside is somewhere in the middle.

I'm not sure its worth doing an experiment to determine the exact neck tension differences. I think I could figure out how to measure insertion forces with my set-up but not extraction forces.

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slimjim
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PostPosted: Wed Sep 12, 2012 3:11 am    Post subject: Re: Neck Tension Reply with quote

A couple variables that contributed to the high extraction forces.

1) these were Barnes TSX bullets with the bands on the bullet shank. I think that helps the case neck grip the bullet better. Note, I still get high extraction forces even with Berger VLDs.

2) the 0.300 button was too small. I should have been using a 0.302. I was trying to replicate the neck tension of my Lee Classic Handloader Die by matching the outside neck diameter of its resized cases. Turns out the Lee Classic has a tapered neck and I was measuring at the thinnest part.

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SingleShotLover
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PostPosted: Wed Sep 12, 2012 4:47 am    Post subject: Re: Neck Tension Reply with quote

It sounds like you might have cases that vary in neck thickness. It isn't unusual to notice a difference in "pull" if this is the case. Whenever I buy brass, the first thing I do (after running them over the expander button to make sure they are round and trimming to length) is turn the necks. I do this with the cutter set to just clean up the necks, not to do a wholesale thinning. You will find that some cases will have the cutter marking only one side rather than cleaning the whole neck. This is a pretty good indication that that case is of uneven thickness throughout the entire body and it is best to keep those for just practice. Just find a case that has a neck thickness to your liking, set your cutter depth accordingly, and run the new cases through. This will ensure that your neck tension stays the same for each case, keeping pressures even and usually improving accuracy.

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slimjim
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PostPosted: Wed Sep 12, 2012 5:15 am    Post subject: Re: Neck Tension Reply with quote

Sorry, SSL. Varying neck thickness is not the issue. I've been mic'ing them. Threw out almost 2 dozen cases that weren't within 0.001 from the Remington brass I had been reloading. In the specific situation above it was brand new Nosler Custom Brass and it had been mic'd. It is that first firing that made the step change in neck tension.

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Bushmaster
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PostPosted: Wed Sep 12, 2012 7:56 am    Post subject: Re: Neck Tension Reply with quote

Keep it comin'...I'm listening.

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Donut Slayer
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PostPosted: Wed Sep 12, 2012 2:05 pm    Post subject: Re: Neck Tension Reply with quote

Bushmaster wrote:
Keep it comin'...I'm listening.
me 2

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gelandangan
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PostPosted: Wed Sep 12, 2012 2:49 pm    Post subject: Re: Neck Tension Reply with quote

could that be because the neck is work hardened?
Another thought is minute corrosion within the neck caused rough surface.

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slimjim
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PostPosted: Wed Sep 12, 2012 6:13 pm    Post subject: Re: Neck Tension Reply with quote

gelandangan wrote:
could that be because the neck is work hardened?

If the brass had been fired 10 times, I would think work hardening might have been a consideration. But after one time? I thought not.

gelandangan wrote:
Another thought is minute corrosion within the neck caused rough surface.

I can see this roughness inside the neck even after the first firing. The shine is gone even when I use the ultrasonic cleaner to get all the carbon off.

I'm trying to think of a method to measure the difference in force during bullet extraction. Maybe use a scale measure the force to push the handle up? Other ideas?

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PaulS
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PostPosted: Wed Sep 12, 2012 10:05 pm    Post subject: Re: Neck Tension Reply with quote

I use a brass brush to clean the inside of the necks. It does a great job and the seating tension doesn't change as far as I can tell. I lube the inside of the neck before sizing and then clean them after in my tumbler to remove any oily residue. That is when I clean with the brush - to get any media powder off the neck.

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gelandangan
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PostPosted: Wed Sep 12, 2012 10:58 pm    Post subject: Re: Neck Tension Reply with quote

I stabbed my brass neck into a small container of steel balls with graphite powder mixed in it.
The balls would "paint" on a very thin layer of graphite that promote the projectile to slide in nicely.

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MacD
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PostPosted: Thu Sep 13, 2012 4:40 am    Post subject: Re: Neck Tension Reply with quote

Think elasticity. That new brass is likely to have a lower coefficient of elasticity meaning when stretched it doesn't return to as close to its original size. The fired brass has a higher elasticity but a lower tensile strength due to hardening.

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slimjim
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PostPosted: Thu Sep 13, 2012 5:50 am    Post subject: Re: Neck Tension Reply with quote

I was thinking of trying to measure neck tension with

1) a new case,
2) a once fired case with carbon removed
3) a once fired case with no cleaning

I may need to compare or use the 0.302 neck sizing button vs the 0.300 to see how much difference that makes.

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slimjim
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PostPosted: Thu Sep 13, 2012 5:56 am    Post subject: Re: Neck Tension Reply with quote

gelandangan wrote:
The balls would "paint" on a very thin layer of graphite that promote the projectile to slide in nicely.

I had a long-range shooter once tell me he likes to use moly-coated bullets fro that reason. We would load up a bunch of shells leaving the bullets seated long. Then as his throat wore, he would adjust COAL before the match. He said the pressure to readjust the seating depth varied noticeably until he switched to moly-coated bullets.

I stopped using moly-coated bullets because it adds one more variable to the reloading equation. Plus once you use them, you can't switch back to using regular copper jacketed bullets without messing up your loads performance, e.g., the moly residue in the barrel no affects the non-moly coated bullet.

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Aloysius
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PostPosted: Thu Sep 13, 2012 6:03 am    Post subject: Re: Neck Tension Reply with quote

Maybe you should also measure neck tension between these cases and crimped ones. Lee's advertising that their crimping die makes the differinces in extraction forces smaller, because they are all high after a treathment with the factory crimping die, so the % difference will be smaller.

And just a question: I think the burning rate of the powder used to reload that round will also have a large influence. I think the faster the powder the lower the extraction force (for rounds with all the rest being the same)
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Ominivision1
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PostPosted: Thu Sep 13, 2012 7:27 am    Post subject: Re: Neck Tension Reply with quote

One thing I've found out is years ago I use to resize & prep my brass and reload at a later date. When I received a shipment of new brass I immediately resized, prepped and loaded the components and noticed the bullets going in easier then with brass that was a month or so waiting to be loaded.

I hooked up an old hand held metal fishing scale to the press arm and found out that the new cases resized and prepped and loaded immediately took anywhere from 15 to 25lbs to seat the bullet.

On the cases that were resized and set aside for a month, upon loading the cases it took 45lbs to 60lbs to seat the bullet. Just recently I did the same test with all new cases again and found the same exact results when I did the 1st test years ago.

So it appears to me, if you load your components immediately after resizing, you will have less neck tension. And if you load cases that have been already resized sitting for a month or so, you will have more neck tension.

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