Neck tension???

[186thFCo]

Ok so I picked up a set of Hornady match dies for my 7mm RM and determined which bushing to use as per Hornady instructions on YouTube.

I took a loaded case, meseared the diameter half way down the neck (.310). So I wanted a .002 neck tension minus an extra .001 for spring back and got the .307 bushing.

So I resized some brass and checked the ID (inside diameter) of the case neck and I was expecting to close to .282, .002 smaller diameter of the 7mm/.284” bullet. Instead I got .276, .008 smaller.

Did I miss something? I did everything according to the instructions why would it be .006 off?

 

[918v]

Your bushing funneled the neck. If you measure the OD at the case mouth and at the mid neck, you’ll see a difference in diameter. That’s the nature of brass flow through a constriction.

 

[ShtrRdy]

I have noticed that if the fired brass neck diameter is more than about 0.005" over what you are wanting to size to then the neck sizing step will actually cause a smaller diameter than what the bushing is.

 

[spife7980]

Why not compare one dimension to the same dimension?

You gave the before outside. What’s the after outside?

 

[antithesis]

Measuring the inside dimension is next to impossible without the right tool. A simple caliper is not going to do it. As spife7980 said, measure the outside of a loaded round. Take that dimension and subtract whatever neck tension you want and that is the bushing you need.

Measure in several spots around the neck about 1/3 of the way down.

 

[186thFCo]

Measuring the inside dimension is next to impossible without the right tool. A simple caliper is not going to do it. As spife7980 said, measure the outside of a loaded round. Take that dimension and subtract whatever neck tension you want and that is the bushing you need.

Measure in several spots around the neck about 1/3 of the way down.

So I used a .250-.375 intromicrometer to measure the inside of the neck not calipers. I am a machinist, one of the jobs I do a lot of is making bushing for helicopters so when I hear .002 neck tension my train of thought is that I’m looking for a .002 press fit that I didn’t get. So my question now is a .002 press fit what I’m actually looking for or is what I’m getting (.008 press fit) what I should have?

 

[FishDr]

This is probably not the case at at… and I don’t mean it to be an insult. Is there any chance the case that was resized was a new case? The neck usually runs smaller on a new unfired case.

 

[186thFCo]

These are once fired cases

 

[spife7980]

So I used a .250-.375 intromicrometer to measure the inside of the neck not calipers. I am a machinist, one of the jobs I do a lot of is making bushing for helicopters so when I hear .002 neck tension my train of thought is that I’m looking for a .002 press fit that I didn’t get. So my question now is a .002 press fit what I’m actually looking for or is what I’m getting (.008 press fit) what I should have?

Then measure the inside of a fired case so that you get some equal comparisons. And it is better to take the before/after measurements on the same piece of brass in the same locations. Do several pieces as well.

You’re gonna find out that the necks aren’t even and your disjointed comparison measuring techniques are going to give you issues.
Case in point: https://www.snipershide.com/shooting/threads/neck-tension.6941594/

Edit: also, once fired in your chamber or from someone elses? Wont effect the loaded round diameter but still helpful data. Providing more, complete information will only get you more, complete answers.

 

[918v]

Look at the neck. Does it look straight like a factory new neck?

 

[FishDr]

Then measure the inside of a fired case so that you get some equal comparisons. And it is better to take the before/after measurements on the same piece of brass in the same locations. Do several pieces as well.

You’re gonna find out that the necks aren’t even and your disjointed comparison measuring techniques are going to give you issues.
Case in point: https://www.snipershide.com/shooting/threads/neck-tension.6941594/

Spife … if that was intentional … that is hilarious!
And exactly what I have found … an endless do-loop of continual frustration trying to control neck tension to an exact science. But I’m still working on it .

 

[antithesis]

So I used a .250-.375 intromicrometer to measure the inside of the neck not calipers. I am a machinist, one of the jobs I do a lot of is making bushing for helicopters so when I hear .002 neck tension my train of thought is that I’m looking for a .002 press fit that I didn’t get. So my question now is a .002 press fit what I’m actually looking for or is what I’m getting (.008 press fit) what I should have?

When we talk about neck tension, we are talking about the diameter of a loaded neck vs. newly sized neck. To get the ID not considering spring back, your calculation would be (bullet diameter) + (neck wall thickness x 2) - bushing ID. Using the info from your original post, your necks should be 0.0155 without spring back, so your necks are likely 0.0145 when considering spring back which is about what Lapua, Nosler, and Peterson brass are.

 

[186thFCo]

Ok I see, so I had the wrong idea of what neck tension is. I should be all set than.

 

[spife7980]

Ok I see, so I had the wrong idea of what neck tension is. I should be all set than.

Worth a read if you havent already
https://www.accurateshooter.com/technical-articles/reloading/neck-tension-not-just-bushing-size/

 

[918v]

Ok I see, so I had the wrong idea of what neck tension is. I should be all set than.

No, you had it right.