Neck tension calculation

[Slim13]

I searched older posts, but couldn't find the answer I'm looking for. I'm trying to measure my neck tension using the following process:

Cartridge: 260 Remington

1) measurement of the outside case neck (with a bullet seated) =.293
2) measurement of neck wall thickness (Lapua Brass). Using my calipers I had measurements of .016,.0165,.017 (I settled on .0165 for my calculations)

.293-.033=.260
.260-.264= -.004
.004 neck tension

Is my answer correct?

 

[mileletron]

Yes.

 

[RonAKA]

Yes, you are correct but ONLY IF your measurements are accurate. It is hard to measure neck thickness accurately with calipers. Yes, you can get a number, but is it right. You really need a micrometer that measures to the 1/10th of a thou. First measure the bullet. They are not all a nominal 0.264". I am currently working on modifying a die to give me the right fit for a 6.5 RM (0.264 same as your 260). The current bullets I have in my box range from 0.2630 (Speer) to 0.2642 (Lapua Scenar). So if you want to know the real neck tension you have to subtract the actual bullet size, and really need to be measuring to a tenth.

I have measured the neck thickness of my cases which are Remington by using calipers (not a good idea), and got almost exactly the same range as you - 0.016 to 0.017". But I am getting a final neck diameter of as high as 0.2960" with the 0.2642 Lapua bullet. Seems like either your numbers or mine are wrong....

I suspect your calculated inside diameter based on measured wall thickness may be wrong, and you could have close to zero tension. What did it feel like when you seated the bullet?

 

[243winxb]

Measure neck before and after seating a bullet.

 

[Slim13]

Yes, you are correct but ONLY IF your measurements are accurate. It is hard to measure neck thickness accurately with calipers. Yes, you can get a number, but is it right. You really need a micrometer that measures to the 1/10th of a thou. First measure the bullet. They are not all a nominal 0.264". I am currently working on modifying a die to give me the right fit for a 6.5 RM (0.264 same as your 260). The current bullets I have in my box range from 0.2630 (Speer) to 0.2642 (Lapua Scenar). So if you want to know the real neck tension you have to subtract the actual bullet size, and really need to be measuring to a tenth.

I have measured the neck thickness of my cases which are Remington by using calipers (not a good idea), and got almost exactly the same range as you - 0.016 to 0.017". But I am getting a final neck diameter of as high as 0.2960" with the 0.2642 Lapua bullet. Seems like either your numbers or mine are wrong....

I suspect your calculated inside diameter based on measured wall thickness may be wrong, and you could have close to zero tension. What did it feel like when you seated the bullet?

The seating stoke felt like it always does. I use Forster micrometer seating dies and a nice gentle stroke ensures accuracy in seating depth. It's hard to describe because it's a "feel thing" but I do feel the seating resistance. I've got a great load developed that shoots lights out. I'm just curious about measuring tension in the interest of consistency. I'm a hunter not a Bench Rest or competition shooter.

 

[DeadEyeDick45]

Measure neck before and after seating a bullet.

I like this answer. After all, OP did say he's not a bench rest shooter.

 

[chkunz]

See reply #4 above.

 

[Shynloco]

I like this answer. After all, OP did say he's not a bench rest shooter.

As a Benchrest Shooter, I believe that's the best response I've seen yet. Measuring before and after will give you the most reliable answer to the posed question without "assuming" spring back and neck thickness are identical in all you casings. It will also show you potential variations in finished cartridges which is why I use this method to double check how much variations I have, if any. Of course, if you are going to use thickness measurements of resized but unloaded necks and then overall neck thickness once loaded, you better have "quality" tools to obtain accurate measurements. And I'm not talking inexpensive Calipers and Ball Mics.

Alex

 

[mikecr]

I'm trying to measure my neck tension using the following process:
is my answer correct?

No. Your merely assembling interference fit.
Nothing to do with tension..

 

[Slim13]

I'm grateful for all of the responses here. Although I'm not a competitive benchrest shooter like some on this site, I do pay attention to the techniques mentioned here and put some of them to use. 4 months ago I never reloaded a cartridge in my life. Last week I developed a load that gave me the group above. I just want to understand all the variables so I can consistently recreate this load. Thanks again

 

['Freak]

Thanks to the albeit narrow but still flats on the width of caliper blades and how those flats will contact the inside curvature of smallish diameter cylindrical walls, ya plain ol’ can’t use them thangs expecting to get an accurate actual case neck wall thickness measurement.
Example: Your caliper measured .0165” average neck wall thickness 2x plus a .264” bullet diameter = .297”, that’s .004” greater than your measured .293” loaded round neck outside diameter; ain't possible.

 

[noload]

Goes to show you neck tension is a feel thing...not a number thing.

 

[dmoran]

Measuring the amount we size the neck in relationship to its stretched measurement of the loaded round.
Example:
0.290" = measured neck diameter after sizing, but before seating a bullet
0.293" = measured neck diameter after the bullet is seated
0.003" = difference - neck tension in inches

Nice target !.!.!
Donovan

 

[mikecr]

Measuring the amount we size the neck in relationship to its stretched measurement of the loaded round.
Example:
0.290" = measured neck diameter after sizing, but before seating a bullet
0.293" = measured neck diameter after the bullet is seated
0.003" = difference - neck tension in inches

Probably seems picky for the sake of that, but this notion of tension is merely a misnomer for interference fit.
And, it's important that folks understand the difference.

For a neck sized no more than seated bullet bearing(normal bushing sizing), the scenario described here does not produce .003" of tension. It produces no more than sprung back from .293", which is ~.001" at most, to ~.292".
That is, if you were to pull the bullet, the neck would spring back to ~.292", and that differential is what was gripping the bullet.

Sizing to .290", or all the way down to .280", makes no immediate difference to tension. When you seat a bullet in this excessive interference fit, the bullet simply expands the neck like any expander (except bullets are not supposed to be used for this).
Then pulling a bullet from this condition will always show that seating expanded the excessive interference fit, and that the neck springs back the same amount (~.001") from cal.

Go ahead and do it. Grab any loaded round with partial neck sizing, measure the neck, pull the bullet, measure the neck again.
With this, you'll see your spring back, which is all that was gripping a bullet. It's what firing pressure will overcome to release a bullet.
ANY downsizing beyond this is just wasteful & bad energy added. It does no more than overwork necks, cause higher seating forces, contribute to runout, and add to your tension variance (because it changes ductility).

FL sizing of necks is another matter all together, that I've already gone through. I would never, ever, do that.
Also, actual tension cannot ever be directly expressed in interference fit. This, because spring back itself is meaningless to tension until applied as force x area. So 1thou of X spring back area to 22cal is way different than 1thou of Y spring back area to 30cal. Then you have firing pressures needed to overcome an area gripped, as applied to different hoop areas, supported by different shoulder angles. It would take more pressure to expand a 22cal neck(hoop) supported by a 40deg shoulder, than a 30cal neck supported by a 20deg shoulder.
So we can apply shortcuts & misnomers till the cows come home. But we're a long way from measuring and knowing neck tension.

 

[savagedasher]

I have a question wouldn't Neck tension change with bullet diameter ?
I know it does for interference fit with steel .
Larry

 

[SheepDog]

Yes, neck tension changes with diameter. Assuming clean brass and the same interference fit The contact surface is Pi times the Diameter so the amount of friction would be more for a larger diameter bullet.

 

[mikecr]

Friction has absolutely nothing to do with tension.
It's hopeless here...

 

[RonAKA]

Sizing to .290", or all the way down to .280", makes no immediate difference to tension. When you seat a bullet in this excessive interference fit, the bullet simply expands the neck like any expander (except bullets are not supposed to be used for this).
Then pulling a bullet from this condition will always show that seating expanded the excessive interference fit, and that the neck springs back the same amount (~.001") from cal.

I partly agree.

The neck returns to original size if you stay within the elastic behavior part of the stress strain curve for cartridge brass. Within that range more strain produces more stress and thus more gripping force. This is shown from the part of the curve below from the origin o to a line. However after you start to yield you get more strain and stress but a non linear rate. The brass is yielding, and we do this to brass all the time. The brass initially starts fairly weak, and point a on this graph is quite a bit lower on a relative basis. When the stress is released, you go back down the dashed line and incur that permanent set, which is represented by the size the ID gains from seating the bullet and pulling it out again.

But, if you size that brass down, and cause it to yield again, and then seat another bullet, the stress strain curve is different. It has the same slope but the yield point is pushed up. The proportional limit is extended, and you get more spring back, and more gripping force. Without annealing you get tougher and tougher brass and more gripping force.

So that is why we anneal. If we don't the gripping force keeps increasing every reload.

 

[Joe R]

No. Your merely assembling interference fit.
Nothing to do with tension..

mikecr nailed it.