Brass "springback" question....

[Char-Gar]

I know nothing, zero, zip, nada about metals in general and cartridge brass in particular, but I have noticed something that puzzles me and I am certain some of you have the answer.

Cartridge brass seems to "springback" when expanded and not so when compressed. To be specific, I am talking about the case necks of 30-06 cases. I have been doing some fiddling and measuring and this is what I have found;

1. Measure the neck expanding plug and then the ID of the case that has been passed over it and there will be about .001 "springback". i.e. a case passed over a .310 expander will have a .309 ID. Use a .311 expander and the ID will be .310.... I have done this with expanders from .307 to .314 and each time the expanded case measures .001 less than the expander. We are talking +- .0002 here and I am certain this number will vary with make of brass and how many times it has been sized. In this instance, they were once fired R-P cases.

2. Measure the diameter of the neck area in a FL sizing die and then the OD of a case ran through it and there is less than .0002 if any difference. I am using a Lyman Shell Resizer hand die in an arbor press. I repeated this with an older C-H FL size die and got the same results.
So, expand a case neck and it "springsback". Compress a case neck and it stays put. Why is this?

Please don't give me a lecture on how to use precision measuring tools as I know how and these numbers are correct. I used good Starrett Small Hole Gages and a good Starrett 1" micrometer that reads out to .0001. I have 50 plus years of experience in their use

If you know the answer, please don't use "tech talk", terms and concepts I would not understand. If I knew those terms, I probably wouldn't need to ask this question.

[country gent]

As metal gets compressed it gets denser. ( same wieght less area) when expanded it becomes less dense ( same weight more area)/ all metals have some amount of spring to them lead being one of the few "dead" metals. Also remeber when sizing a case down in caliber the neck gets thicker when sizing up it gets thinner. Also aiding in this is a brass cartidge walls are not "straight" but tapered. At the case head they may be .020 - .025 thick and at the mouth only .010-.012 thick. This taper aides in the punches used to form cupps and cases to be removed from them easily. It also allows for thicker brass where heat and pressures are more and thinner more ductile where the case needs to seal the chamber. Under pressure of many firings brass will "flow and change thickness. To put this in perspective after 12-14 firing of my swift brass with 50 grn bullets I have to neck turn the cases as the necks have thickened enough so a bullet no longer slips into a fired case. Annealing will help to lighten the spring effect and soften the brass so it will seal and work better also.

[RickinTN]

I'm not really sure why you're not getting any "spring back" when you size or decrease the neck diameter. It should be about the same (.001" or so). I've never tried the expander measurements and springback, but your results are about what I would expect. In using Wilson or Redding bushing dies it is widely accepted practice to use a bushing .001" smaller than your desired outside diameter to account for this.

Rick

[uscra112]

Interesting. I've never tried to do that measurement, since I have never used bushing-type dies. Just out of curiosity, does a case that's been sized and expanded 2-3 times give the same result? Wondering if the initial sizing of an annealed neck gave you minimal springback, but the sizing workhardened it enough that you got springback when expanding.

[RickinTN]

I think that an annealed case would respond differently than a work-hardened case but by how much I really don't know. I never shot formal benchrest, but did learn my accuracy loading technique from those guys. I settled on my bushings .003" smaller than loaded round neck diameter. This, when the springback is accounted for, will give .002" neck tension. This works very well for me for ammunition that will be transported. Many benchrest shooters if not most load at the bench while shooting. They "adjust" for conditions. I understand many of their rifles are set up to not have to neck size at all. They take advantage of the springback to release the bullet under pressure and then return to a diameter that will hold a bullet. Very tight tolerances and much closer than I want to deal with for an informal target or varmint rifle. I have been able to achieve <1/4" groups @ 100 yds. consistently if the rifle is up to it, but the serious benchresters would chuckle at that. I do have bushing dies and bushings for 308 Winchester, and intend to use them in my cast bullet endeavors, but haven't had the opportunity to work with the 308.
Take care,
Rick

[44man]

Yes, brass will spring back and the more it is worked the more you will have.
Rifle brass necks can be annealed to reduce it.
Brass is funny. I had to take apart a broken and ruined original muzzle loader to save parts and found the spring that locked the patch box was brass, plain brass, not some spring material.
I never expect a spud to expand necks to spud diameter because I still want case tension on a bullet.
If it was not for brass and the way it expands and returns, we would still be shooting muzzle loaders.