Neck Bushing/expander mandrel question

[Timon]

I have been shooting for many years and reloading for about four. In my past reloading I always though my neck tension was around .002”, but I never really got into it in a whole lot of detail. Here is my question. If you use a neck bushing to establish your neck tension, is there not spring back in the brass. I have read that this is around .001”. My thought is that if you are trying to get a neck tension of .002” and you use the precise neck bushing to do that, won’t you lose .001” off that to take you to .001” neck tension. Based on this should you use bushing that should give you .003” tension, then allow for the spring back with the ultimate tension setting of .002”. I would think that the same would apply to expansion mandrels also. Perhaps I am overthinking this, but I want my neck tension to be correct. Was wondering what others thoughts are on this. Do you take spring back into consideration or not. Thanks for any input.
Scott

 

[Straightshooter1]

Do I take springback into consideration to get the neck tension I'm after? Absolutely. Since I use an expander mandrel to get the neck tension I'm after, I'll use one that's large enough to account for the springback and measure after sizing the neck to confirm I'm getting what I want.

 

[mikecr]

It's actually more like ~1/2thou springback, depending on cal, thickness, and neck/hoop elasticity.
You should add expansion in your sizing plan, as springback continues a bit over time.
Better to have these forces falling to lowest energy inward, than outward.

 

[Mikemontminy]

What you need are gauge pins to measure. They can be bought individual or a full set.

 

[Ned Ludd]

When you squeeze down a fired case at the neck using a bushing die, the neck will open up slightly as it is withdrawn from the bushing. This is the "springback" to which you are referring. In my hands, this typically equates to about half a thousandth or so. Also be aware that commonly-used steel bushings are not exact in terms of their stated diameter. For example, if you were to four or five 0.336" steel bushings and test them all on the same Lot# of brass, you might get measurements ranging from .3355" to .3365" with the different bushing that were all supposedly 0.336". I guess that's why they don't add an extra zero after the "6".

In contrast, when you open up a case neck that has been sized down with a bushing to some diameter smaller than the mandrel, the neck contracts slightly as the mandrel is withdrawn. Again, in my hands this is usually about half a thousandth or so. The important point is that the "springback" when using a mandrel to open up a case neck works in the opposite direction from what is observed when using a bushing to squeeze a neck down.

Between the two processes, it is not a bad idea to have a few different bushings within the useful size range for a given cartridge (neck diameter), as well as a few different mandrels. For example, I have found that with Lapua .308 Win Palma brass, if I want approximately .002" neck tension (interference fit) from a bushing as the sole sizing step, I need a 0.336" bushing. If I want to size the necks down with a bushing first, then open them up with a mandrel as the final sizing step, I need to use a 0.334" or 0.335" bushing first; i.e. one that is slightly yields smaller neck I.D. than to which the mandrel will subsequently open the necks up. That way, the mandrel is actually doing some work when opening all the necks. Due to the springback, the mandrel I would use to obtain ~.002" neck tension in the bushing/mandrel brass prep scenario would be 0.3065", or ~.0015" under bullet diameter. I have bushings for .308 Win dies ranging from around 0.333" to 0.338", and a range of mandrels from 21st Century in .0005" increments from .3060" on up that allowed me to experiment with various combinations to find the right pairing of bushing/mandrel to achieve the desired neck tension. If you assume springback to be somewhere between .005" and .001", you probably don't need more than two or three different bushings/mandrels for a given cartridge to find the right combo.

 

[Timon]

What you need are gauge pins to measure. They can be bought individual or a full set.

They are ordered, just waiting on them to arrive.