Neck Tension Effect on Accuracy

[Stoneke]

I have been working with my Winchester 70 Ext Weather in 7mm RM over the past several months and have been perplexed with changes in group size. After carefully breaking-in the barrel, I found a very accurate load of 68.6 H1000 in Nosler cases with a Hornady 162 ELD-X bullet and F215 primer. My 3 shot groups were 9/16 to 3/4 inch.

During later range sessions I found that groups had opened up to 1 1/2 to 2 inches.
Looking back trying to analyze what changed, I realized that I had switched to Forster bushing dies, and that I was carefully setting the neck tension at .002. Comparing that tension to my initial Redding FL dies, I saw that my polished expander ball was giving me .004 tension. So, would that increase in the initial neck tension produce better burning of the powder and better accuracy? Nothing else has changed - charges, cases, bullets all weighed and segregated and necks turned on the cases.

I can't get to the range for several days, but I'm intending on loading several rounds with tighter neck tension than .002. Any thoughts on this dilemma???

 

[bigedp51]

Your comparing a redding full length die with a Forster bushing bump neck sizing die. Meaning two very different sizing methods.

At the Whidden custom die website they tell you that non-bushing full length dies produce the most concentric ammunition.

At Whidden they also sell expander ball kits with five expanders decreasing from bullet diameter to .004 smaller.

A runout gauge and a neck thickness gauge may help in solving your problem.

If a bushing die reduces the neck diameter .005 or more it can induce neck runout and it is recommended to reduce the the neck in two steps.

I prefer the Forster full length benchrest dies with the high mounted floating expander. The neck of the case is still held by the die when the expander enters the case neck. Meaning the expander can't pull the necks off center and greatly reduced neck runout.

On the flip side of this my Forster bushing bump neck sizing die produces more neck runout than any other die I own. Many say this is because the die does not fully support the case body like a full length die does. And also reducing neck diameter .005 or more with off the shelf factory rifles.

 

[Stoneke]

I have an inside micrometer gauge and a Hornady Concentricity Tool. I have not seen a significant difference in bullet concentricity with either the Redding or Forster dies. I am using a Forster Ultra Micrometer Seating die. Bullet run-out is kept to .001 or less. It is interesting to learn that the touted bushing dies can induce more case run-out. I intend to carefully load another batch with the Redding FL die and see where my accuracy sits.
Thanks for your thoughts.

 

[243winxb]

With unturned neck and bushings , no expander = variation in neck tension.

 

[Stoneke]

Win243: Yes, I understand your stated principle. However, with the bushing die, all of my necks were turned and the correct bushing was used to produce .002 neck tension.

 

[243winxb]

More neck tension will change when the bullet first moves. The pressure spike , when the bullet contacts the rifling , would be at a different time.

Very slow burn rate powders benefit from more neck tension.


We can guess, but testing is the only way to know.

 

[Stoneke]

Win243: Thanks, I suspect that this recent change in neck tension is my culprit. Agree that testing should answer the question. Wish the weather was going to cooperate. Will post my findings.

 

[243winxb]

I find FL sizing is more accurate then neck sizing .

My 243 Redding FL sizing S bushing die works best when the bushing sizes only 1/2 of the neck. Takes a few firing to work best.

 

[Rack]

Tagged

 

[bill123]

Any chance you switched lots of H-1000?

 

[Stoneke]

No, same canister of powder.

 

[Greyfox]

You didn't mention how many reloads you are running per case, but you should also check the brass for work hardening of the necks. I have had this effect my accuracy and ES, requiring re-annealing of the case necks. I anneal my brass at 5-6 reloads. I generally use neck bushings at .002' under the bullet diameter and use an expander. It feels like a light seating pressure that can be felt for consistency. The bullet should not move when pressed heavily against a wood board. Runout is kept under .002". Depending on the specific rifle, I will use FL sizing, or neck sizing only with a body dye resize to .002" under headspace when bolt lift resistance is detected. My standard for LR hunting loads is .25-.5MOA five shot groups with ES under 12FPS. I prefer Lapua or Norma brass that is sized to minimum spec and chamfered. Neck thickness is very consistent. My preference in dyes are Redding S (bushing) dyes except for .223 where I like the RCBS FL X dye with Lapua brass.

 

[Stoneke]

I discovered that a group of cases needed to be annealed when they wouldn't adequately bump size. The range session before that showed poor accuracy. I have since been attuned to annealing, as required. My velocity had dropped a little with these cases that were in need of treatment also. I intend to load batches of both H1000 and Retumbo, each with neck tensions of both .003 and .004 to see what happens. I have lots of both Hornady and Nosler brass which have been fully prepped and weight sorted. Won't be for a couple of weeks though.

 

[Mikecr]

Since switching from FL neck sizing to partial neck sizing, you have an opportunity to learn about tension. Go ahead an bushing size a length of neck no greater than seated bullet bearing. Seat and measure the loaded neck diameter. Then pull the bullet and measure what the neck springs back to. That spring back, against seated bearing area, is your bullet grip (tension).
It will likely be around .0005 to .0008".
You could swap bushings to cause any interference fit greater than this, repeat the test, and there will be no change(spring back is what it is). So adjustment of tension means adjustment of either length of neck sizing, or a change of spring back itself (annealing or work hardening).
If you do this testing, you'll learn what most never do.

FL sizing of necks causes max tension, and max variance of it, with no real control over it. It brings donut area into tension, and also binds bullet base-bearing junction to cause far more/uncontrolled grip than simple force x area.
This is no good, and why partial length bushing sizing has potential for better results(if you know what you're doing). You only need to develop with a plan, that includes a given tension.

As far as runout,, different subject all together. But I've yet to have an issue with runout from use of bushings.