BRASS SPRINGBACK

[PMA3]

The general consensus seems to be that Lapua brass will spring back between .0005"-.001" after sizing or expanding. BUT, the TIME factor also comes into play. Has anyone found any articles, posts or other info on how long(minutes-hours days) it takes for the brass to reach it's FULL spring back.
The numbers may seem insignificant for many shooting disciplines, but for 1000 YD BR, the difference between .001" and .0015" bullet hold (neck tension) could be a big deal. This would effect your timing for sizing, expanding (if you perform this step) and bullet seating. Maybe this is too OCD but it's food for thought.

 

[hogpatrol]

Not sure what key word to use but somewhere in these forums, there's a thread exploring that topic.

 

[JBO]

The general consensus seems to be that Lapua brass will spring back between .0005"-.001" after sizing or expanding. BUT, the TIME factor also comes into play. Has anyone found any articles, posts or other info on how long(minutes-hours days) it takes for the brass to reach it's FULL spring back.
The numbers may seem insignificant for many shooting disciplines, but for 1000 YD BR, the difference between .001" and .0015" bullet hold (neck tension) could be a big deal. This would effect your timing for sizing, expanding (if you perform this step) and bullet seating. Maybe this is too OCD but it's food for thought.

http://forum.accurateshooter.com/th...t-have-an-effect.3769977/page-3#post-35978995

 

[LHSmith]

Go to the Accurate Shooter HomePage, in the Search box- type in "seat tension vs. time"

 

[Webster]

The general consensus seems to be that Lapua brass will spring back between .0005"-.001" after sizing or expanding. BUT, the TIME factor also comes into play. Has anyone found any articles, posts or other info on how long(minutes-hours days) it takes for the brass to reach it's FULL spring back.
The numbers may seem insignificant for many shooting disciplines, but for 1000 YD BR, the difference between .001" and .0015" bullet hold (neck tension) could be a big deal. This would effect your timing for sizing, expanding (if you perform this step) and bullet seating. Maybe this is too OCD but it's food for thought.

Nice short article on stress relaxation on cartridge brass. This is new to me. Not my area of metallurgy. Not sure how to interpret the 100% starting point? Looks like a small reduction in stress at room temp.

http://www.copper.org/applications/industrial/DesignGuide/performance/stress03.html

http://www.copper.org/applications/industrial/DesignGuide/performance/temper03.html

 

[PMA3]

I sized (10) 6 X 47 cases yesterday at 3 pm and measured the outside neck diameter, the turned neck and subtracted to get a neck hole diameter. I measured the outside neck diameter today at 9 am and the average spring back in the 18 hour period was .0002". I am going to check them over the next week or so and see if the spring back changes and by how much.....the case necks were first annealed on a Bench Source.

 

[Webster]

I sized (10) 6 X 47 cases yesterday at 3 pm and measured the outside neck diameter, the turned neck and subtracted to get a neck hole diameter. I measured the outside neck diameter today at 9 am and the average spring back in the 18 hour period was .0002". I am going to check them over the next week or so and see if the spring back changes and by how much.....the case necks were first annealed on a Bench Source.

The articles I posted say that springback isn't much of an issue if the starting stress is below 20%. That's why stress relaxation tests start with 75-100% stress.

 

[CJ6]

I think that the thickness of your "turned" necks will have an effect on your springback/tension also. Thicker or thinner necks may have more or less springback.

 

[/VH]

I sized (10) 6 X 47 cases yesterday at 3 pm and measured the outside neck diameter, the turned neck and subtracted to get a neck hole diameter. I measured the outside neck diameter today at 9 am and the average spring back in the 18 hour period was .0002". I am going to check them over the next week or so and see if the spring back changes and by how much.....the case necks were first annealed on a Bench Source.

Thanks for doing this, I appreciate your efforts and am interested in your results.

Brian

 

[Brians356]

A factor is how work hardened the brass is, i.e. how many times fired and sized without annealing.

 

[Iowa Fox]

Springback will make you tear your hair out. I usually get the cases ready to drop powder and if I have time let them sit for two days and measure again. Always one or two oddballs in the bunch. Annealing helps but doesn't totally eliminate a case or two changing. I've talked to other guys and they get the same thing try as hard as they can also. Its just the metallurgy of a particular case. One guy I know used to cull the offenders and he always did very well.

 

[UltraBR30]

Neck wall thickness, annealed or not annealed, sized in or expanded out, and brass cycles, all can create variables that will effect spring back rates and comparisons between them.
Philip

 

[PMA3]

These Lapua cases have been fired 5 times and annealed after every firing on the Bench Source. All necks turned to
.0105"-.0106" (.267" neck using .2433" dia. Barts) and sized in a custom die without the expander. SB has varied so far (3 days) from .0001"-.0004" (for only 10 cases), thus showing the different metallurgy makeup as stated above. The hardest part is in trying to measure the EXACT same spot on the case necks with the Starrett 1" mic. Operator error, for me, could easily account for a .0001"-.0002" difference, thereby making the results of this test about one step above worthless.
I think I found a better measuring method.....using plug (pin) gauges. I have these in .0002" increments from .2400"- .2430" and will do some future testing after sizing using these gauges.....it should be able to keep the results within .0002". As Iowa Fox and Philip stated above, this could end up being a task where you are continually chasing your tail.

 

[Brians356]

Here's some interesting data I just experienced. Starting with a once-fired 7x57 Mauser case, I ran it through a 257 Roberts FL die (essentially just necking it down) then measured the neck ID (with pin gages) and OD (with an outside mic) at various places (% are rough estimates of distance, from case mouth to shoulder junction, where the pin gage was obstructed):

(Sorry for the justification, "edit" shows proper whit space, but it gets clobbered after I "save".)

After 257 Roberts FL sizing:

ID: 10% .245, 30% .244, 50% .243, 90% .242, 100% .241
OD: 10% .277, ________ 50% .277

So the neck OD was ~cylindrical, the ID narrowed progressively (.004" total) from mouth to shoulder.

Next I ran the "257 Rob" case through a 6mm Rem FL die, and made similar measurements:

ID: ___________ 50% .235, 90% .232, 100% .231
OD: ___________ 50% .269

Now the ID tapers rather more abruptly, changing .004" from 50% to 100% pass through, and there was a definite donut at 90%.

Next I expanded the neck over a Sinclair turner mandrel w/ OD .2409", then checked ID dimensions again:


ID: 10% .241 _____________ 90% .240, 100% .239

Note that the neck "spring back" from the expander was ~0.0" near the case mouth, and ~.0015" at the "donut". (The .240 gage was obstructed by the donut, but the .239 bypassed the donut with room to spare, so the donut is somewhere in the middle.)

So prepping the neck for turning did not move the wall taper (and donut) to the outside, and did not leave the neck ID cylindrical. What ill effect this will have on neck turning I have not yet determined. But I did not anneal these cases at all, so my next experiment will be to anneal after 6mm FL sizing and before the expansion, to see if I can minimize the spring back from the expander - leaving the ID closer to cylindrical for neck turning.

 

[ireload2]

I think if you measure the outside of the necks there will also be a hump on the outside at the base of the neck next to the shoulder.

the shoulder is a much larger stronger area of the case and it is will not move with an expander very easily. Yet it does move and some of the "donut" really is on the outside.
I found this on factory 6mm Rem brass when I tried to size it with a Lee Collet Neck sizer. As you can imagine the collet sizer could only squeeze on the hump at the donut and never touched the rest of the neck.

>>>So prepping the neck for turning did not move the wall taper (and donut) to the outside, and did not leave the neck ID cylindrical. What ill effect this will have on neck turning I have not yet determined. But I did not anneal these cases at all, so my next experiment will be to anneal after 6mm FL sizing and before the expansion, to see if I can minimize the spring back from the expander - leaving the ID closer to cylindrical for neck turning.<<<

 

[Brians356]

In looking at FL dies' effects on neck (RCBS 257 Rob FL, Redding 6mm Rem FL) I noticed the 257 Rob left the neck OD cylindrical, but the 6mm Rem die did not. So I looked closely at those two dies' necks (probing with pin gages) and discovered something interesting: The RCBS FL dies I own all have cylindrical neck IDs, and the Reddings all have slightly tapered necks (~.0010" to .0015" smaller at mouth than base of neck.)

Since most if not all factory cases have tapered neck walls, the Redding die will accommodate that by sizing the thicker portion of the neck slightly less. But once you turn necks to straight walls, the Redding FL will leave the ID tapered (even ignoring any disparate spring-back.) Not sure what effect that might have, but there it is.

Also, I noticed the RCBS FL dies' neck IDs are only .018" to .020" larger than nominal bullet diameter, so they're sizing down necks a lot more than many of us would prefer (no news here!) OTOH my Redding 6mm Rem FL dies' neck ID is .025" larger (at the mouth) than a bullet, meaning that a neck turned to only .012" (and probably even .013") wall thickness would not be sized sufficiently to grip a bullet!

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[TAJ45]

Good info all and I'll probably be utilizing in the semi near future but WOW, is my head starting to hurt.......Hoo Boy.........I'd best stock up on Excedrin.

 

[Brians356]

the shoulder is a much larger stronger area of the case and it is will not move with an expander very easily.

Also consider that when you attempt to expand the neck at the shoulder junction, you are working against the considerable reinforcing buttress of the shoulder. You are not just pushing on the inside of a cylinder there, but trying to compress the shoulder along its axis. A buttress is fairly effective at resisting compression (witness the Gothic cathedrals still standing after over half a millennium.)

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[ireload2]

Yes, the steeper the shoulder angle more of the expander force can be transferred to the upper end of the case body.
While the shoulder will resist the expander force, the base of the neck is weaker and will tend to push a bulge to the outside just above the neck to shoulder intersection.

Also consider that when you attempt to expand the neck at the shoulder junction, you are working against the considerable reinforcing buttress of the shoulder. You are not just pushing on the inside of a cylinder there, but trying to compress the shoulder along its axis. A buttress is fairly effective at resisting compression (witness the Gothic cathedrals still standing after over half a millennium.)

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[Brians356]

Yes, the steeper the shoulder angle more of the expander force can be transferred to the upper end of the case body.
While the shoulder will resist the expander force, the base of the neck is weaker and will tend to push a bulge to the outside just above the neck to shoulder intersection.

I see that even before expanding. Last night I formed a case from 7x57 -> 257 Rob -> 6mm Rem, and carefully measured the neck. Right at the neck/shoulder junction there is a slight bottleneck, the neck OD is ~.001" smaller than .050" closer to the case mouth, where there is a slight donut that swells the outside as well as the inside. From there the neck walls get gradually thinner toward the mouth, the difference in thickness between the donut and the mouth is ~.005"!

This suggests the best way to deal with necks and donuts when forming down from larger necks is to employ a proper ream die and reamer. I've never seen that setup, but I would assume the ream die squeezes the neck OD into compliance, and the reamer is controlled and properly centered to produce reasonably uniform neck walls. (I may have an overly optimistic impression of how it works.)

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