Why does some brass stretch more than others?

[AMMASHOOTA]

I just got finished resizing some once fired Lapua. All the brass was initially trimmed before firing. i measured the shoulder on all the fired cases and they consistently got a 0.0015 bump.

when i measured the OAL after sizing, 11 of the 26 pieces of brass were about 4-6 thou longer than the others. what could account for this?

all brass was processed exactly the same out of the box. i have not adjusted any of my tools.

 

[John Kielly]

You could have caused some of that yourself, too. For instance, if some cases get different treatment during manufacture, then their various dimensions mightn't be the same to start with. Once the cases are fireformed once, the external dimensions are uniformed (more or less) & that is reflected in the OAL now.

I don't final trim until at least one fireforming - just make sure they're under chamber length. Note from my profile, that I'm mainly an open range target shooter & that mighn't be the best call for all disciplines.

 

[Cheechako]

IOW, when you size a case, you are squeezing it down to a uniform ouside dimension. Brass is squirted out the front end 'cause it has no place else to go. Visualize squeezing an inflated ballon.

That's the technical explanation.

 

[Boyd Allen]

Next time measure them after fire forming, before you size them. I believe that you will find that the differences happened as they were fire formed. Do all of your un fire formed cases chamber with a little feel at bolt close? If your fit was determined by the cut on the shoulder when neck turning, and you used a stop to set the length of your cut, without first trimming the unturned cases to a uniform length, the "headspace length" of the cases would vary with their neck lengths, even if your tools were not adjusted differently. An unformed case has very little contact area with the chamber at the shoulder, just at the neck shoulder junction. If there is any variation in fit, it will show up as length as the case is driven forward by the force of the firing pin and primer detonation. As Al mentioned lubrication will can also have an effect. I think that the difference that you see will not be a problem with the formed cases, so pick a trim length that cleans up the shortest and you should be fine from that point on. With uniform bump, case lengthening should be minimal and more uniform. One more thing, normal case "headspace" measuring tools are of little use for un formed .220 Russian cases (because of where they contact the case), and die settings have to be different, if you bump before turning. As much as I disagree with using bolt close to gauge bump for formed brass, I think that it is the best way for un fire formed brass that is to become PPC.

 

[Cheechako]

Ammashooter

If you are agonizing over differences in case length of 4 to 6 thou during the forming process, you are being way too serious. That much difference can come from something as simple as a burr on the neck, or holding your caliper at a slight angle. After your cases are fired a couple of times, trim them to a uniform length and shoot them. But, you're going to be chasing small differences in length right up to the day that the cases are worn out and you toss them in the trash. Then you make a new batch.

JMHO

Ray

 

[AMMASHOOTA]

Ammashooter

If you are agonizing over differences in case length of 4 to 6 thou during the forming process, you are being way too serious. That much difference can come from something as simple as a burr on the neck, or holding your caliper at a slight angle. After your cases are fired a couple of times, trim them to a uniform length and shoot them. But, you're going to be chasing small differences in length right up to the day that the cases are worn out and you toss them in the trash. Then you make a new batch.

JMHO

Ray

i appreciate the info. i'm not agonizing, per se, just trying to understand what is happening and why. i sort of figured if i applied the same technique consistently, i would get consistent results. that does not appear to be the case (sorry for the pun).

 

[Cheechako]

I'd say that 4 to 6 thou during forming IS consistent.

 

[AMMASHOOTA]

Next time measure them after fire forming, before you size them. I believe that you will find that the differences happened as they were fire formed. Do all of your un fire formed cases chamber with a little feel at bolt close? If your fit was determined by the cut on the shoulder when neck turning, and you used a stop to set the length of your cut, without first trimming the unturned cases to a uniform length, the "headspace length" of the cases would vary with their neck lengths, even if your tools were not adjusted differently. An unformed case has very little contact area with the chamber at the shoulder, just at the neck shoulder junction. If there is any variation in fit, it will show up as length as the case is driven forward by the force of the firing pin and primer detonation. As Al mentioned lubrication will can also have an effect. I think that the difference that you see will not be a problem with the formed cases, so pick a trim length that cleans up the shortest and you should be fine from that point on. With uniform bump, case lengthening should be minimal and more uniform. One more thing, normal case "headspace" measuring tools are of little use for un formed .220 Russian cases (because of where they contact the case), and die settings have to be different, if you bump before turning. As much as I disagree with using bolt close to gauge bump for formed brass, I think that it is the best way for un fire formed brass that is to become PPC.

i appreciate the advice. this a .308, not a PPC. no neck turning. i specifically chambered this with 0.002 headspace. bolt closes on unfired brass without any 'feel' to it.

i guess my concern was having half the brass sitting at an OAL of 2.003, and the rest at 2.008-10. couldn't figure out why such a discrepancy for cases that weren't being worked so hard.

maybe i should start drinking again / more...

 

[Boyd Allen]

If you look at just about any unfired brass, its "headspace" is well under chamber minimum. When more shallow shoulder angles "take a run at" the chamber shoulder, on the first firing, with all that room, they wedge forward, making their necks longer, and most of the case is held in that position by the friction created by the pressure of combustion. At the back of the case, if the pressure is high enough, the case is stretched back to the bolt face, causing a very slight thinning just in front of the thickest part of the case wall, in front of the head. This is not a problem, as long as subsequent sizings do not push the shoulder back excessively. My advice it to trim your cases to the longest lengh that will square up your shortest case, and don''t worry about it.

 

[AMMASHOOTA]

You've just isolated your problem. .002 headspace. My advice is to rechamber, correctly.

al

rilfe shoots too good right now to mess with. this is not a BR rifle. i will use it to hunt with as well as punch paper. i originally set the headspace to 0.0 and had second thoughts about taking it out into the desert -- that's when i punched it out another 1.5 thou. i understand your concern but it is shooting 1/4 MOA and i am happy with that.

 

[AMMASHOOTA]

If you look at just about any unfired brass, its "headspace" is well under chamber minimum. When more shallow shoulder angles "take a run at" the chamber shoulder, on the first firing, with all that room, they wedge forward, making their necks longer, and most of the case is held in that position by the friction created by the pressure of combustion. At the back of the case, if the pressure is high enough, the case is stretched back to the bolt face, causing a very slight thinning just in front of the thickest part of the case wall, in front of the head. This is not a problem, as long as subsequent sizings do not push the shoulder back excessively. My advice it to trim your cases to the longest lengh that will square up your shortest case, and don''t worry about it.

thanks for your advice. i will do that.

 

[Larry Elliott]

Most factory new brass can be run into a standard FL die without any lube except on the inside and outside of the neck. The bodies are smaller than dies before firing, and expand in all dimensions to fit/fill the chamber of the rifle. Even chambered with zero headspace that new brass will chamber likely chamber easily. Trying to do everything right is a great thing, but there are more important things than all the cases being absolutely the same length. Bullets seated with minimum runout, primers properly seated, powder charges within reasonable weight limits, and a lot of other stuff to worry about. Just make sure that the cases don't get too long so that the necks can't expand to release the bullets. That can really screw up accuracy and maybe the rifle.