Neck Tension Conundrum

[RTK]

I have been thinking a lot about neck tension and what most feel it should be. If the neck tension is consistent on all shells loaded, what does it matter if the neck diameter is 2-3-8 or 10 thou under bullet diameter.

 

[Lefty7mmstw]

I have been thinking a lot about neck tension and what most feel it should be. If the neck tension is consistent on all shells loaded, what does it matter if the neck diameter is 2-3-8 or 10 thou under bullet diameter.

It matters because if you are more than a couple of thou. under bullet diameter the brass will tend to stretch offset a bit and you will be introducing run-out into your ammo.

 

[AZShooter]

IMO trying to press a bullet into a case with a difference much past .004" is asking for problems. The neck could be pushed out of alignment causing excessive bullet runout translating to poor accuracy.

You didn't factor neck wall thickness into your theory. Using thin (.010") or thick (.014") neck walls can change neck tension complicating the issue on how much to generate with your dies.

Best to stick with what has shown to work. If you can go with .001" or .002" difference between sized and seated diameters you will get best accuracy for single shot application. You may need more tension if the rifle is a repeater especially if it has high recoil. A crimp may be needed in some cases, the 375 H and H comes to mind. Typically RCBS full length dies create a .003" neck tension difference.

 

[Bart B]

When the spread in neck tension (called release force in the ammo industry) increases, so does muzzle velocity. It's typically about 20% about the average amount. Lighter is always better than heavier for accuracy purposes. But too little won't hold the bullet in place during normal handling.

A .001" smaller case mouth diameter (after sizing) than bullet diameter is about right for most uses for bullets up to 200 grains.

 

[BrentM]

Interesting. I found my RCBS full length die was around .003 mark as well. I have bushings that stepped down to .002. My case neck loaded is .294 and the bushing is .292 that I use. Fired empty is .296.

Having said all that I tried a smaller bushing for more tension and the rifle didn't seem to care for it that much and I noticed a bit more kick and cratering in the primer. Guess that is what these are refering too, more pressure to get the bullet moving.

 

[jsthntn247]

loaded round of .294 and fired at .296 is a tight chamber, wished my full custom was that good.

 

[Trickymissfit]

I've done some serious experimenting with neck tension in 22 and 6mm calibers, and one thing I learned fast was that there will be a sweet spot in the amount of tension that will greatly make a difference is your extreme spread in velocity. One thousandth of an inch took my ES from double digits (close to twenty fps) down to about seven or eight fps on a .223 case. The 6mm Remington also seems to be touchy on this issue alone, but not as much as the smaller case was. Now I starting to experiment with my 6/250AI.
gary

 

[BrentM]

loaded round of .294 and fired at .296 is a tight chamber, wished my full custom was that good.

Oh, I had no idea if it was good or not. It is a stock Savage 116 6.5-284. I was just curious about expansion from sized, to loaded, to fired.

 

[RTK]

Thanks for all the comments.
Here is the situation, in the past I have been neck sizing only with an occasional bump back after 3-4 firings. In my readings everyone now is saying to FL size every time. Unfortunately when I do this with my FL die it induces random run out. I've tried all the tricks loosen this tighten this, many different lubes inside the neck, polish the expander etc. Then I decided to remove the expander ball, concentricity is excellent. (necks have been turned to knock of the high spots only)
Took 25 of these to my range yesterday. (8 thou under bullet diameter) The ES and SD were no better or worse than before and neither was the accuracy.

 

[Bart B]

Took 25 of these to my range yesterday. (8 thou under bullet diameter) The ES and SD were no better or worse than before and neither was the accuracy.

I think that your case mouths are way too small if they're .008" under bulet diameter.

Unless you die's neck has bushings sized .002" under loaded round neck diameter or is that size already, they're not being expanded enough. That's what the expander ball does. Standard full length sizing dies make sized necks way too small without the expander ball being pulled back up through them.

 

[BrentM]

I think that your case mouths are way too small if they're .008" under bulet diameter.

Unless you die's neck has bushings sized .002" under loaded round neck diameter or is that size already, they're not being expanded enough. That's what the expander ball does. Standard full length sizing dies make sized necks way too small without the expander ball being pulled back up through them.

I was thinking the exact same thing but thought I would let someone smart answer this. The general rule I see out there is .002 neck tension. If you look at Lee, RCBS etc they all say .001-.003. I was encourage to run .002 for enough tension to hold the bullet securely for a repeater. I assume I am doing things correctly at this point. Perhaps you could give it a shot.

Here is what I do:
1. measure the loaded neck...example. .294
2. .294-.002 = .292
3. Insert bushing. Neck size. example .292 is the sized neck.
4. check chamber neck space, measure fired case. Example .296*
*did not know how good or bad this is for me. just knew it was enough to allow the neck to expand some. I assume it is actually a bit larger since the case grows and retracts a bit.

With a neck bushing I don't use the expander ball. I use the small end to punch the primers only. With a FL die I have to use the ball, the neck is way tight, such as yours.

 

[RTK]

Bart, You are one of the people that made me decide to give the FL sizing a go.
Yeah, I,m sure it is too tight but that huge random run out induced from the expander ball drove me nuts..
I order a redding FL bushing die and I will remove the expander ball. I had the builder of this rifle make the neck pretty tight (it's on the spec sheet) but not so much that I "had" to neck turn. The die is coming with a couple of bushings so I should have no problem hitting from .001-.003.

 

[RTK]

One more comment on run out. I haven't done any definitive testing for myself yet but have read that acceptable run out related to accuracy is much higher than I would have imagined.
This stuff gets under ones skin and then you have to keep chasing that devil in the pursuit of perfection...........

 

[Bart B]

This stuff gets under ones skin and then you have to keep chasing that devil in the pursuit of perfection...........

Now that you're on the paths of righteousness, you'll find it's a lot more fun to chase angels instead of that mean old man dressed in red.

Regarding bullet runout on rimless bottleneck cases, depending on the tool used and how it's set up, a given bullet wil easily show .002" spread across all tools and methods. None of them measure runout holding the case the same way it fits the chamber when it's fired. With the tools available today, I'd guess than runout up to 1% of bullet diameter's acceptable.