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Neck Tension
Discussion regarding the reloading of ammunition and tuning of loads for accuracy
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chambered221
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PostPosted: Sun Jan 13, 2013 10:26 am    Post subject: Re: Neck Tension Reply with quote

The spammer is more interesting perhaps !!! Poke

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chambered221
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PostPosted: Sun Jan 13, 2013 10:36 am    Post subject: Re: Neck Tension Reply with quote

Seriously now ......... I've read some articles about neck tension in the past that only reinforced the too many variables concept.

Some shoot better with press fit bullets, others prefer a looser fit.
The thing that has always confused me is the press fit. No matter how much you shrink the neck it is always opened back up to the bullet diameter. Yet... the amount of under sizing can/will make a difference.

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slimjim
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PostPosted: Sun Jan 13, 2013 11:05 am    Post subject: Re: Neck Tension Reply with quote

Vince wrote:
Be interested to see pics of the force scale mate, and how you set it up. I have always relied upon the standard neck sizer that comes with the dies.

I wonder what sort of difference there may be in velocity and POI on the target? Also, what effect this had on accuracy, if any.

Thanks for the interest, Vince.

Here is how I set-up to measure the force required to extract a seated bullet. Now that I see the picture, the scale is working off a lever so the force measured is not the force of extraction (neck tension) which should be higher.

This test is not dependent on what type of die a reloader uses. It only measures the force of extraction between a new and fired case which has been sized to the same neck dimension. If I size the necks smaller, this force will go up (noticeably for the fired brass). If I size the necks larger, the force will go down. At the current neck size (.302 bushing for a .270), I'm going slightly beyond the 50 pound limit of this scale.

I could do some testing just for fun to see what the affect is on group size and point of impact. From my experience, I believe it will be measurable but I'm sure its significance is at longer ranges. For now, I use fired, resized brass when the POI is important, e.g., hunting with shot opportunities beyond 100 yards.



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Last edited by slimjim on Sun Jan 13, 2013 11:28 am; edited 1 time in total
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slimjim
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PostPosted: Sun Jan 13, 2013 11:12 am    Post subject: Re: Neck Tension Reply with quote

chambered221 wrote:
No matter how much you shrink the neck it is always opened back up to the bullet diameter. Yet... the amount of under sizing can/will make a difference.

There is some elasticity in the brass so that when the bullet is pulled, the neck diameter of the case contracts. After I pulled these bullets, I could not insert them back into the cases by hand. I still had to press them back in.

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Elvis
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PostPosted: Mon Jan 14, 2013 12:57 am    Post subject: Re: Neck Tension Reply with quote

very nice set up thankyou for sharing that slim.

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Vince
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PostPosted: Mon Jan 14, 2013 2:50 am    Post subject: Re: Neck Tension Reply with quote

Thanks Slim...a good repeatable exercise.

When I say I use the standard neck sizer, there will always be variations in the actual neck sizing button between manufacturers and even machines used to produce said button. But the setup you have there will negate those variations unless you are chopping and changing between die made by different manufacturers and then only data from each individual manufacturer would be considered repeatable. You wouldn't be able to compare them to each other with any real accuracy.

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slimjim
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PostPosted: Mon Jan 14, 2013 7:46 am    Post subject: Re: Neck Tension Reply with quote

I could run a test with every component the same except new brass vs fired brass and measure the difference in POI and accuracy. It will give me something to do this summer.

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slimjim
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PostPosted: Sat Apr 13, 2013 7:34 pm    Post subject: Re: Neck Tension Reply with quote

Well, I've tried several attempts to measure the effects of crimping on neck tension. I went out and got myself some Lee crimp dies. Using the crimp feature built into my Redding bullet seating dies is too finicky. If everything isn't perfect the crimps are inconsistent and I found could bulge the neck. The Lee crimping dies do not require a consistent case length to function. I did some tests with my six8 and .270 Win with the Lee equipment. I used 150gr Berger VLDs in both brass cases. The Lee crimping die is a collet that presses inward on the case and has enough leverage to deform the bullet with the crimp. I can't say I have learned anything definitive but I thought that I’d share what I have found and see what comments you might have. I used fired brass that was sized and ultrasonically cleaned so did not have any powder on the inside of the neck. The longer necks on the .270 did result in higher neck tension than the shorter neck of the six8. Note, there is a mechanical advantage with the press so the true force to extract is higher than what I recorded.

The average extraction force of the 8 bullets from the six8 was 9 pounds without a crimp (4) and 11.1 pounds with the crimp (4). Force was about 20% higher with the crimp but variation in force did not get signnificantly better with the crimp.

The force to extract the bullets from the .270 was higher but my first attempt caused nothing but confusion. I loaded 4 and crimped 2 of them with a heavy crimp. Force varied from 24 to 44 pounds with no correlation to the crimp. The crimp was strong enough to deform the bullet and leave a groove where the crimp occurred. So I loaded up 10 more and crimped 5 with a light crimp. The ones without a crimp averaged 32 pounds (22 to 43 lbs). The ones with a light crimp averaged 25.5 pounds (21 to 32). It made me think that the crimp may have been bulging out the neck below the crimp causing a reduction in overall neck tension. So I loaded and crimped 4 more with a heavier crimp. I got nice consistency out of the first three, 34 to 36 pounds, then the fourth one slipped out with only 12 pounds of force (lowest ever for the .270 and it had a crimp?). I’m not eager to run more cases through this drill without figuring out what is happening. Going forward, I will continue to crimp the six8 just because I did get a slightly higher force and that can’t hurt with an autoloader. For my bolt action .270 , I’m not going to crimp when I’m using grooved bullets like the Barnes TTSX and Hornady GMX. The grooves in these all-copper bullets do increase neck tension as the neck grips the grooves. The lead-core VLDs have been very consistent shooters without the crimp (1/2 MOA). I may continue working with the crimp on the legacy lead-core bullets just to learn more. Eager to hear your thoughts.

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Pumpkinslinger
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PostPosted: Sun Apr 14, 2013 1:51 pm    Post subject: Re: Neck Tension Reply with quote

The last time my neck crimped the chiropractor was able to fix me right up!

Sorry about that.

Slim, good effort and info there!

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gelandangan
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PostPosted: Sun Apr 14, 2013 2:41 pm    Post subject: Re: Neck Tension Reply with quote

+1, but I go to physio..

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slimjim
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PostPosted: Sun Apr 14, 2013 2:58 pm    Post subject: Re: Neck Tension Reply with quote

Maybe I should visit a witch doctor. I was expecting more consistency in the results. It was disappointing that there wasn't a direct correlation to increased and more consistent neck tension when crimped.

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gelandangan
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PostPosted: Sun Apr 14, 2013 3:43 pm    Post subject: Re: Neck Tension Reply with quote

Slim, the pressure exerted by the neck could be considered as minor as compared to the pressure subjected tot he whole system when the powders are combusted.
Although uniform neck grip does increase in the uniformity of the end velocity, it is but one of many elements that contributed to the cause.
In fact, in the old days when boolits are paper patched, the whole neck tension is the result of the fit of the paper to shim the gap between the neck and the boolit.
Yet there are historical records of amazing shots with them.

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slimjim
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PostPosted: Sat Apr 20, 2013 2:34 pm    Post subject: Re: Neck Tension Reply with quote

Was at the work bench fiddling around so took four .270 cases, sized them, seated bullets, crimped them, and pulled them. Average extraction force was 32.3 pounds with a std dev of 5. Then for grins and without touching the necks, I reloaded all four cases, did not crimp them, and pulled them again. I wanted to see how much the neck tension would be reduced after a bullet had been through the neck once. Do you want to take a guess what the average neck tension was? Come on guess.


32.1 pounds but the std deviation doubled to 10.

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slimjim
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PostPosted: Sat Apr 20, 2013 2:47 pm    Post subject: Re: Neck Tension Reply with quote

All right, why not do this one more time. Seat bullets back into the untouched cases but crimp them this time. Results.

Extraction force dropped a little bit down to 29.1 (3rd time a bullet had been seated into the neck) but the std dev dropped back down to 6.

So far it looks like the .270 has about 3 times more neck tension than the 6.8 most likely due to the longer case neck and I use a 0.002" tighter neck bushing when I resize the case. It also appears the the std deviation is improved with the crimp but on the .270 it has enough neck tension that it doesn't really increase neck tension. Again these are with lead-core bullets. I would not crimp an all-copper bullet like Barnes.

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MacD
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PostPosted: Sat Apr 20, 2013 3:57 pm    Post subject: Re: Neck Tension Reply with quote

The variables that relate to brass are both within and outside of our control. The alloy itself will in part determine elasticity and resistance to work hardening. The annealing at the factory between the steps necessary to produce the finished case plays a part. The relative thickness of the case wall will affect neck tension. Finally the diameter of the bullets also may vary slightly. We live in an anolog world which we measure digitally. Thank the maker for variabilty. It makes it all interesting doesn't it.?

Try the same bullets in the same cases and see if extraction force decreases consistently. I am betting it won't.

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