Brass springback over time

[Oldbloke]

When you measured the second time, was it a hot day? Or very cold day first time?

 

[BoydAllen]

Sounds like the OP has some more experiments to do. I will be interested in his results.

 

[searcher]

Hello all,

My once fired, annealing .308 lapua brass was all full length resized down 0.002 to 1.479 BTO. I measured almost every piece and they had no runout.

They were then cleaned in a tumbler with warm water.

That was 1 week ago. I have come back to reload them and they mostly now measure 1.480, with a fair few at 1.481.

I was under the impression that springback happens very quickly after sizing. But I'm seeing a relaxing of the brass to the tune of up to 0.002 after a week.

Would the warm water have caused this springback? Or is it just a function of time? Or is this just a common phenomenon for newer brass? It is rather frustrating, as whilst most pieces have relaxed to 1.480, there are some that didnt and some that went all the way back to 1.481. It would interesting to understand what happened....

I'm travelling to compete abroad for the first time next year, so need to load up a lot of rounds ahead of time. Id rather go with rounds that have the same headspace.

Thanks!

I have had that happen - and have posted about it on this site. There are non-believers that the brass continues to spring back after a day or two - but it definitely can - and does. I once loaded up a fairly large quantity of ammo and fired some of it the next day. All was fine. A month later, at the range, I noticed every bullet was getting pushed into the case upon loading (A/R). Has to pull the entire batch. At that time, I wasn't annealing after every firing and that no doubt contributed to the problem a bit.

 

[FTR_Mojo]

When you measured the second time, was it a hot day? Or very cold day first time?

Funny you say this. It was a particularly hot day. My room is air conditioned, but not 24 7, so it can get warm in there occasionally. But we're only talking a 6 centigrade difference. Not 20!
I measured today (its cooler) and many were back to 1.479...
Surely it can't be that simple!
I'm doing another batch this week, so am going to try dwelling even longer and washing in cold water.
I will let you know if I find any difference.
The bottom line might be I bump 3 thou rather than 2. Just for security. As I often do hihh volume brass prep well in advance.

 

[Straightshooter1]

Hello all,

My once fired, annealing .308 lapua brass was all full length resized down 0.002 to 1.479 BTO. I measured almost every piece and they had no runout.

They were then cleaned in a tumbler with warm water.

That was 1 week ago. I have come back to reload them and they mostly now measure 1.480, with a fair few at 1.481.

I was under the impression that springback happens very quickly after sizing. But I'm seeing a relaxing of the brass to the tune of up to 0.002 after a week.

Would the warm water have caused this springback? Or is it just a function of time? Or is this just a common phenomenon for newer brass? It is rather frustrating, as whilst most pieces have relaxed to 1.480, there are some that didnt and some that went all the way back to 1.481. It would interesting to understand what happened....

I'm travelling to compete abroad for the first time next year, so need to load up a lot of rounds ahead of time. Id rather go with rounds that have the same headspace.

Thanks!

First . . . brass will spring back whether they're not annealed or properly annealed (like an AMP might do). Some springback happens quickly after sizing, but typically, it take ~24 hrs or so for the brass to complete its springback. How much spring back will happen will depend on its hardness and how much movement it's gone through in the sizing operation. Warm water will surely cause the brass to expand a little while its warm, but not "cause" springback otherwise.

You didn't say just how you anneal your brass. Do you use an AMP or flame anneal. If you flame anneal, can you describe the detail of your operation. To me, it really sounds like you're not getting a good enough anneal, which is common for those who've subscribed to annealing to 650°F.

 

[FTR_Mojo]

I anneal on an AMP. I did an aztec analysis, and have use the code it generated. So I think I'm doing the right thing there. I could run another one and check I suppose.

 

[FTR_Mojo]

Seems like there's a few factors at play here. Ambient temp, warm water in the tumbler, potential dwell time. I'm going to change things up for this next batch and see if I get an improvement in stability.

Really appreciating all the responses. Given me some good things to think about and some excellent anecdotal evidence of brass movement or lack thereof.

 

[Oldbloke]

Funny you say this. It was a particularly hot day. My room is air conditioned, but not 24 7, so it can get warm in there occasionally. But we're only talking a 6 centigrade difference. Not 20!
I measured today (its cooler) and many were back to 1.479...
Surely it can't be that simple!
I'm doing another batch this week, so am going to try dwelling even longer and washing in cold water.
I will let you know if I find any difference.
The bottom line might be I bump 3 thou rather than 2. Just for security. As I often do hihh volume brass prep well in advance.

Suggestion.
Measure 5 at ambient.
Then do it again after 15 minutes in the fridge. But do it quick or they will warm up.

 

[dellet]

I don’t see where you have said how you are measuring. Does the tool stay assembled between measurements, and or does it need to be rezeroed after sitting the two weeks between measurements? Yes to either or both will be part of the problem.

Have you ever measured before and after tumbling?
Tumbling is in many ways a stress relief and also has a peening effect. Thousands of light hammer blows to the surface of all the cases. That is another data point you might want to check.

If none of the brass chambers tight after it is fired, but does after sizing and waiting 2 weeks, it’s not spring back. It’s actually grown past its original length. Not too many ways for that to happen.

 

[Chris Edmondson]

I expand using a mandrel as a last step. Id be surprised if that was causing the shoulder to move. But worth a test.

If expanding does cause the shoulder to move, I'm inclined to suspect a donut. For that reason, I've been shoulder bumping after neck sizing.

 

[M-61]

I am I guess odd man out.
I don’t wash and don’t anneal.
Vibrating tumbler.
Never ran across a problem as described above.
I don’t use that washing, stainless pin method. Why? Don’t like getting the brass wet (of course you can dry it) but I never felt jewelry finish brass tightened one group ever.
Annealing? Well I’m not a metallurgist and I just don’t like the whole idea. Scientific? Not a chance. And I do understand that Some brass (22 hornet for example) may split a neck easily. But I don’t shoot it,
I’m happy with the groups I shoot and I actually think what I shoot is easy on the brass. I merely stay with what works for me in my bench rest rifles.

 

[Doom]

Hello all,

My once fired, annealing .308 lapua brass was all full length resized down 0.002 to 1.479 BTO. I measured almost every piece and they had no runout.

They were then cleaned in a tumbler with warm water.

That was 1 week ago. I have come back to reload them and they mostly now measure 1.480, with a fair few at 1.481.

I was under the impression that springback happens very quickly after sizing. But I'm seeing a relaxing of the brass to the tune of up to 0.002 after a week.

Would the warm water have caused this springback? Or is it just a function of time? Or is this just a common phenomenon for newer brass? It is rather frustrating, as whilst most pieces have relaxed to 1.480, there are some that didnt and some that went all the way back to 1.481. It would interesting to understand what happened....

I'm travelling to compete abroad for the first time next year, so need to load up a lot of rounds ahead of time. Id rather go with rounds that have the same headspace.

Thanks!

@Straightshooter1 has the issue pegged. Brass has springback and it will have a lag time to reach an equilibrium.

To the issue related to the enlarged brass chambering. Cases will typically reach equilibrium with the chamber that is less than the actual chamber size. This is due to springback after firing. Seldom will it equal the chamber size and if it does it will begin to be hard to extract or you will have "clickers". Even when the case quits growing it is likely it is smaller than the chamber.

As to seeing the long term springback the amount that the brass wants to spring back is related to how much it is resized and it will do so opposite the direction it was sized. IN the situation where the case neck is sized the case neck is sized less than the desired dimension then expanded with an expander or mandrel to the desired dimension. Doing this results in the case neck trying to decrease in diameter so bullet tension isn't lost. Unfortunately in the case of the shoulder it will always grow longer.

So I know what you are thinking, did I size it 0.002 or 0.001 inches. Well, in all the sizing I don't remember myself, so you have to ask yourself "can I shoot the difference". (aplolgies to "Dirty Harry"!

 

[Ackman]

Springback is something I'd never experienced, never even thought about it. But a few years ago after shooting some fairly hot 6BR loads, Lapua brass, I put that stuff aside for awhile. After maybe 6 months, or more, that brass wouldn't chamber completely. After checking everything, it wasn't the lower part of the case, it was the shoulder / body dimension that had expanded.. FL sizing fixed it.

 

[FTR_Mojo]

Suggestion.
Measure 5 at ambient.
Then do it again after 15 minutes in the fridge. But do it quick or they will warm up.

Out of curiosity. I took your advice and gave this experiment a go. 2 of each brass of three different shoulder lengths.
After 2 hours in the fridge, 1 of six was 0.001 shorter.
After 1 hour on top of a hot surface, 2 of six were 0.001 longer.
Back at room temperature, all measured where they started.

I'm not going to worry about ambient temperature changes. I will use warm instead of hot water to clean though.

 

[Doom]

Springback is something I'd never experienced, never even thought about it. But a few years ago after shooting some fairly hot 6BR loads, Lapua brass, I put that stuff aside for awhile. After maybe 6 months, or more, that brass wouldn't chamber completely. After checking everything, it wasn't the lower part of the case, it was the shoulder / body dimension that had expanded.. FL sizing fixed it.

When the brass is resized the body is the first part that is resized and depending on the chamber diameter that will determine how much the fired shoulder is pushed forward. Part of the final stroke reforms the new shoulder from the body and part of the old shoulder is reformed to the neck.

If your cases body diameter isn't growing much on firing there is less resizing of the shoulder so less springback.

 

[Oldbloke]

Out of curiosity. I took your advice and gave this experiment a go. 2 of each brass of three different shoulder lengths.
After 2 hours in the fridge, 1 of six was 0.001 shorter.
After 1 hour on top of a hot surface, 2 of six were 0.001 longer.
Back at room temperature, all measured where they started.

I'm not going to worry about ambient temperature changes. I will use warm instead of hot water to clean though.

Interesting result. I expected more. Thx for posting.

 

[rlporter507]

have you considered a difference in your measuring tool???? IF digital, was it zeroed each time, is it a quality digital tool, is the battery fresh???
You could try a quality dial tool to eliminate this possibility. Maybe its not the brass?

 

[47WillysGuy]

Hello all,

My once fired, annealing .308 lapua brass was all full length resized down 0.002 to 1.479 BTO. I measured almost every piece and they had no runout.

They were then cleaned in a tumbler with warm water.

That was 1 week ago. I have come back to reload them and they mostly now measure 1.480, with a fair few at 1.481.

I was under the impression that springback happens very quickly after sizing. But I'm seeing a relaxing of the brass to the tune of up to 0.002 after a week.

Would the warm water have caused this springback? Or is it just a function of time? Or is this just a common phenomenon for newer brass? It is rather frustrating, as whilst most pieces have relaxed to 1.480, there are some that didnt and some that went all the way back to 1.481. It would interesting to understand what happened....

I'm travelling to compete abroad for the first time next year, so need to load up a lot of rounds ahead of time. Id rather go with rounds that have the same headspace.

Thanks!

Neck Measurements
Date Processed Date Measured Lot Original As Found Day Spread Dem Dif
12/7/2025 12/12/2025 -15 0.2878 0.2877 5 0.0001
11/4/2025 12/12/2025 -08 0.2879 0.2878 38 0.0001
11/3/2025 12/12/2025 -09 0.2879 0.2879 39 0.0000
10/21/2025 12/12/2025 -14 0.2878 0.2878 52 0.0000


Bump Measurements
Date Processed Date Measured Lot Original As Found Day Spread Dem Dif
11/4/2025 12/12/2025 -08 1.5549 1.5565 38 0.0016
10/21/2025 12/12/2025 -14 1.5546 1.5568 52 0.0022
12/7/2025 12/12/2025 -15 1.5548 1.5572 5 0.0024
11/3/2025 12/12/2025 -09 1.5535 1.5563 39 0.0028
I was under the impression that spring back happens very quickly after sizing. But I'm seeing a relaxing of the brass to the tune of up to 0.002 after a week.
0.002" approximate spring back is what I also found.
Would the warm water have caused this spring back?
I do not believe so. My annealed case are air cooled. Case sizing lube is removed with Sprayway.
Or is it just a function of time?
A very short time seems. Less than 5 days for Lot -15.
Or is this just a common phenomenon for newer brass?
No, Lot -08 is 6 years old and has 20 firings.
It would interesting to understand what happened....
My theory is that the spring back happens because the body and should are not fully annealed, but the case neck is. My propane torch turns the case neck neck red, really red, but not orange, there is no color change on the body and shoulder. Neck dimensions are consistent even after 52 days.
Good luck on your shoot.