# Neck tension

#### Straight Shooter

##### Well-Known Member
How do you measure neck tension?

#### Rick Richard

##### Well-Known Member
OD of loaded round (neck) minus OD of sized neck equal neck tension. Bushing is .02 less than OD of loaded round measurement.

I think I said that right.

#### Mikecr

##### Well-Known Member
You can't.
What Rick described is INTERFERENCE fit.
And seating forces are not tension either, but seating FRICTION.

Neck tension is the gripping force applied to an area of seated bullet bearing (in PSI).
We currently have no means to measure it.

#### Rick Richard

##### Well-Known Member
You can't.
What Rick described is INTERFERENCE fit.
And seating forces are not tension either, but seating FRICTION.

Neck tension is the gripping force applied to an area of seated bullet bearing (in PSI).
We currently have no means to measure it.
Call it what you want, but most folks identify Neck Tension with what I describe.

#### Straight Shooter

##### Well-Known Member
Mike, then does neck tension or gripping force or seating force make a difference and is the only practical way to measure it is by interference fit?

#### Mikecr

##### Well-Known Member
Neck tension does make a difference, but interference fit is not a direct way to measure it, nor a certain way to obtain it..
Let's say you set your neck interference to 10thou under cal for no more area than seated bullet bearing.
Does that 10thou increase neck tension over 2thou interference?
Your seated bullet will simply up-size the neck to counter.
If you then pull that bullet you'll see your necks spring back to only around 1thou (max) interference. Even more likely ~1/2thou.
That's what you get with that effort, plus all the detriments caused by excessive sizing.

Actual tension, being pressure per area, can be controlled through area/length sized.
While seating .300" of bearing in necks, you could adjust sizing length (interference length) from .300" to .200" or .125" for examples to adjust tension. The interference amount which you would be adjusting the length of, need be no more than ~1thou, which the bullet will reset it to anyway.

This affects seating force as well. But it's not a direct measure of tension, and it's comparative value is hard tied to friction.
The friction has to be well normalized.

#### Straight Shooter

##### Well-Known Member
Mike, so is there a certain amount of interference fit I should want?

#### Blacktailer

##### Well-Known Member
Mike, so is there a certain amount of interference fit I should want?
I hear most people shooting for .002-.003 interference but as long as I can push a bullet tip on a loaded round against the bench as hard as I can and it doesn't move, I'm usually happy as long as your dies are not overworking the necks of the brass.
Had a resize die that sized the necks down about .008 before they were pulled over the expander. Led to neck splits on 2nd or 3rd reload.

#### MagnumManiac

##### Well-Known Member
Talking of consistency with IF, the most drastic thing for me, besides annealing after every shot before sizing, is using powdered graphite on the insides of my necks. My ES/SD shrunk doing this and velocities became slightly less.
I HBN coat my solid CNC turned 323g .338” bullets for my 338-416 Rigby, and the velocity in that rarely alters by 8fps, which is the lowest ES of any of my rifles, including my high end comp F-class guns, which average in the low teens.
The only 2 cartridges I size 85-95% of the necks are my 264WM & 300WM.
I find neck length has little to do with Precision. My 25-06 that I use in Hunter comp shoots extremely well with .180-.200” of the neck sized with 115g Berger Hybrids, this just shows that at .003” IF, this is sufficient to keep bullets straight while just kissing the rifling, maybe .002” into the rifling proper.
My 22-250AI is similar, only needs .200” of bullet bearing in the neck.

Cheers.

#### Mikecr

##### Well-Known Member
Mike, so is there a certain amount of interference fit I should want?
The amount I want is 1thou under cal after spring back. This ensures I have full force diametrically, and even that little will be upsized with seating for 22-26cals.
The length I usually begin with(for load development) is 1/2 seated bullet bearing.

My necks are usually at or turned to 12thou thickness, with initial process annealing and later annealing with seating forces fighting for matching. Neck friction is normalized with it's fired carbon film.
I'm sure I could press hard enough or hammer bullets in, but that's never a problem for me. My guns are single shot or converted.
Magazine capacity is not as powerful as cold bore single shot accuracy.

There is a condition where neck tension is greatly increased beyond where I go: when sizing length exceeds seated bearing (like FL sizing of necks). That pretty much automatically works for extreme pressure underbores. But for hunting capacity cartridges, I will never do this.

The gain to be had with neck tension is normally with it's consistency. If you seek low variance of it, you'll head to lower tension, and develop with that. It could then lead to a faster(but case filling) powder, maybe another inch of barrel length. This, if far enough away from lands that lower tension also lowers starting pressure. But I just haven't run into any issue with it & my cartridges.

#### Straight Shooter

##### Well-Known Member
Holy smokes, Magnum Maniac, you are so far ahead of me that I will never catch up to your level. Thank you for the info. It might take the rest of my life to try and understand it.

#### Straight Shooter

##### Well-Known Member
Mikecr, ^^^^^^same to you. Wow.

#### MagnumManiac

##### Well-Known Member
Holy smokes, Magnum Maniac, you are so far ahead of me that I will never catch up to your level. Thank you for the info. It might take the rest of my life to try and understand it.
I’ll give you a hint of WHY what we do in regard to IF changes with length of neck sizing, it is the AMOUNT of friction on bullet release.
Factory ammo is crimped because they use a fast powder to save cost and you will notice a lot of air space. This crimp has been used for decades because it raises start pressure to a level that would be found with a slower powder without the crimp.
This is why a change in FRICTION, either from a crimp, or neck IF, can drastically alter a loads parameters, as does seating depth.
IF stays the same, about .001”, on the bullet shank, but changing it’s length is like fine tuning it, as Mikecr refers to.
I have only ever gone to half bullet OD in IF length on 22 cal, everything else I load for is maybe 70% minimum.
It was once thought that a long neck was conducive to accuracy, this has been debunked over and over, as many rounds these days have less than OD cal necks and are exceptionally accurate.
If you seat into the rifling, or the same amount from it, the only way to tune is with primer/powder and IF length.
Hope this makes sense.

Cheers.

#### nealm66

##### Well-Known Member
Thanks for sharing your knowledge and experience!