Fire formed cases with different shoulder measurements?

J

Jbordi

Guest
I have some fire formed cases in a 7mm rem. mag. that have varying measurements at the datum line. Some cases vary as much as .003 to .004 after firing. These are measured before any sizing is done. Primers are popped out before measuring.

Any ideas on this? Could some of the cases be stretching more than others above the belt? Would this growth show up at the shoulder? Some loads are hotter than others.

This is from a custom barrel.

Thanks
 
I can tell you exactly why you're getting the results you're getting. Because of your headspace. You have excess headspace, period.

It's not "dangerous" except that IF you're a reloader, unless you get it figured out you will experience case separations. Getting case separations are like getting a flat tire on your car, generally very anti-climactic, "no harm done." But once in a while the fates conspire against you. I know a person who's blind in one eye from a simple case separation.

The other side effect is loss of accuracy. You will never achieve consistent accuracy with buggered headspace. It's like having a computer virus..... the comp still WORKS 90% of the time at probably 70% efficiency. Or it's like your truck being out of tune. Bad plugs/wires/rotor. You get used to it. You don't even miss the lack..... until it's fixed.

Anyway, you've got excess headspace. You must find a way to deal with it.

After the smoke clears from people speculating and hollering that "it might be this and that" we can talk about how to fix it.

al
 
Some more details I left out.

Barrel manuf. used a min. spec. reamer on this barrel. I don't have headspace gauges?

Reminton brass is showing the varying dimensions.

Nosler brass used in same rifle doesn't show variance?

I have cut some of the Remingtion brass at the belt to see what is happening on the inside. No sign of thinner brass above the belt.

Thanks

Jarrod
 
I expect that the inconsistent initial length of the remington brass is causing each case to headspace differently in your chamber giving you varying lengths after firing.


There are things you can do but it's up to you to weigh up the effort.
 
Where, on the cases, are you measuring.... from the head to the shoulder, or to the front of the belt? Have you measured new brass? There are no SAAMI headspace dimensions, on belted cases, to the shoulder. (It is measured from the head to the front of the belt.) In several instances that I have measured fired, and compared to unfired, there was a little over .020 difference.

If you are measuring to the cases shoulders, and resize to the longest as fired measurements you will probably notice that after a few firings, that they all start to approach the same measurement. I would see how a case chambers when sized to produce the longest fired case headspace (to the shoulder), and if it is satisfactory, I would leave the die at that setting for the rest. (I would do this each time a set of cases was fired.)

Calibers with thick brass may exhibit more variation in first firing measurements, to their shoulders, as well as variation in the same measurement after sizing. This is likely to be because of variance in the annealing of the brass. If I were loading for one of these, and did not have an annealing machine, I would sort the cases after sizing, and carefully readjust the die for the ones that measured longer at the shoulder.

A couple of years ago, I was helping a friend load a 7mm WSM for an African safari, and we neglected to check the shoulder bump of all of his cases. We won't make that mistake again. There were a few difficult bolt closings in the field. After he returned, we solved the problem with an annealing machine. The same situation has come up with .336 Lapua brass.
 
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Boyd,

I am measuring from head to shoulder. I am only neck sizing these cases. So far all of the cases do chamber with only little resistance.

I do bump back shoulders .001 when needed.

Thanks for info.
 
I would recommend that you fireform with the bullet engaging the rifling. This will establish the shoulder at the correct length for YOUR rifle. I would not set the shoulder back after that and only neck size.

This is the same procedure used in making wildcat cartridges where the shoulder needs to be blown out or the angle changed. I generally use a reduced (starting) load.
 
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Better to fireform with a false shoulder and a mid-to-upper-end load.
 
You could take .300 Winchester brass -run it thru a .338 Win . die to set the shoulder for your chamber .
Then run that case thru a 7mm Rem die to finish forming.
Cut to length.

Then the cartridge headspace will match your chamber head space before firing.

You will have to anneal the necks before turning as the neck springback varies on the hardness.

Glenn:)
 
One question, when you say the primers have been poped out, did you do this with a dies that had a inside mandrel/inside sizer? if so that will strech your brass,,,,,

the wind is my friend,,,,,,,,,,,,,,

DD
 
If you are using a light load, or one that takes up little space in the case body, unequal ignition can reduce the chamber pressure some what.

When I used light loads in my .303, which being rimmed is somewhat like a belted case so far as headspace goes, light loads sometimes did not expand the shoulder completely. This was obvious to the eye, the shoulder looking rounded rather than sharply defined.

Variations in belt dimensions were an early problem with belted magnum cartridges.

When using light loads of fast burning powder in a 7.92 the fired cases became shorter and shorter with each firing and reloading. These were neck sized only using a Lyman 310 nutcracker.

Primers began to protrude.
Not sure but I think these were loaded using magnum primers, the only large rifle primers I could find locally for awhile.

I had figured the primer was pushing the case deep into the chamber setting the shoulder back, and the chamber pressure wasn't adequate to fully expand the case.

I was using mostly Norma brass, and some from Canadian BESA MG ammo.

The loads were not ultra light, some where several gr above 30 gr but I forget the exact weight. Power level was a bit higher than the .30-30.

The fast burning powder may have caused the case walls to grip the chamber walls tightly so primer pocket pressure prevented the case from coming back against the bolt face to reseat the fired primers.
There was some smoking of the necks so it would appear pressure was not high enough, or didn't climb fast enough, to fully expand the neck quickly, with a tad of gas escaping around the neck.

PS
A belted Magnum chamber and headspace may be perfect but variations in belt dimensions can induce an excessive headspace situation, just as thin rims can increase head gap in a .303.

Bolt lug bounce can shorten a fired case. The metal of the lug being compressed then regaining its shape. usually not more than a thousandth of an inch if that.

If not for lug bounce/rebound it would be very difficult to chamber neck sized only cases when reloaded.
 
Jarrod,

Ok..... so you've got some headspace. IMO you've got EXCESS headspace but as with many things, that's a matter of opinion.

So here's how I'd proceed. :) In fact DID proceed just a couple weeks ago. I have a 7MM Mag in with excess headspace. Luckily this one's a Savage so I could "fix it" easily even though we almost lost the lettering!

Here's my progression.

First I want to know how much actual clearance I've got to the belt and is the belt dimension cut correctly..... is it doing it's job? To do this I cut off a case about 1/2" or so above the belt. Clean up the rough cut. Now I iron a hunk of masking tape over the headstamp and trim it cleanly around with a 'zacto knife. Close it in the gun. I'll venture a guess that your rifle bolt will close with no resistance. If it closes easily, iron on another hunk. This layer I'm guessing you'll feel. If it closes now, you must "gauge" the closing. Did it close hard? Did you juuust feel it or is it scary hard like "gall your bolt lugs" hard? Basically what you need to do is play with thicknesses (types/brands) of tape until you feel you've got a nice solid feel and mic it or measure thickness with a caliper.

Point is, with a liddle b'guess and b'golly and some judicious calipering you can establish your actual to-the-belt headspace within a half a thou.

If you've got over .002 of clearance than you're gonna' have a fight on your hands to get good brass.

Know this..... loading bullets "long" and jamming them into the rifling lands WILL NOT fix the problem.

Once you've established #1 that you HAVE excessive belt clearance and #2 that you want to fix it, then I'd proceed as follows;

-First find another die or a mandrel that's a couple calibers larger.

-Now, with new brass 'spander the necks up to say 30 caliber.

-Back your sizing die off a full turn with the lock ring loosened to a slight fit and run the lock ring down onto the press. Set it so's you can turn the die but it's not sloppy. Run the 7MM die down the case to reset a false shoulder. The bolt will not close. Now we're back to b'gollying. Slowly work the die down until the bolt closes. Tighten everything down when you've got it set such that you've got a tight jam-fit. Set it so that you have to heel the bolt closed with your palm. GREASE YOUR BOLT LUGS!!!!

NOW you're set to make some good brass.

Now just LEAVE your sizing die set where it is.

IMO you can't fix the already fired cases. They are what they are. You can keep shooting them. Use them as a gauge, shoot them head-to-head with your other cases. You should be able to get 5-8 firings on your newly made cases before needing to fine tune your die setting. They should shoot well with just a little feel as you close the belt. And at some point they'll tighten up so much that they're uncomfortably hard to close. At this point you'll find out about belted cases and sizing..... you'll find out about the brass kinda' "rolling up" ahead of the belt. You might consider a Willis Collet die. You might decide 5-6-8 firings is enough. You might even decide to ignore all of this and just proceed :)

it's up to you.

But also know this. IF you keep pushing the shoulder back and IF you've got what I suspect is .004 of headspace play behind the belt... you will sooner or later get the aforementioned head separation.

The mechanism is this.

Even though the belt is designed to hold the case back, IT DOESN'T for that .004 or so. The case drives forward until it stops on the belt, (or the shoulder,) fires and STICKS forward in the chamber. It doesn't slide. If it comes back (it will once you're loading hot enough) it comes back by stretching. Ahead of the belt.

do this enough times and the case comes in two.

My suggestion, the method outlined here, is to IGNORE the belt and headspace on the shoulder like a conventional case. Now you've got to work around the belt while maintaining your cases.

I hope this helps you

al
 
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Thank you Al and Old Gunner!

This is great information that will help me.

Jarrod
 
As I remember the 7 Rem Mag head spaces off the ring and not the shoulder.

When I fire form my 25BR Long Brass it is common for me to see a length delta of .003 or so on the first firing. Usually it takes two and sometimes even three cycles for every thing to get equal in a 50 case lot.
 
Herzo said:
I would recommend that you fireform with the bullet engaging the rifling. This will establish the shoulder at the correct length for YOUR rifle. I would not set the shoulder back after that and only neck size.

This is the same procedure used in making wildcat cartridges where the shoulder needs to be blown out or the angle changed. I generally use a reduced (starting) load.
Click to expand...

Usually the bullet is simply driven into the neck by the firing pin if there is headspace and you try to reply on the bullet jam to hold the case against the bolt face. Creating a false shoulder at the neck is the only sure way of getting a consistent fire form. With only bullet jam you get case stretching and a weakened web area.
 
The manufacturers apparently count on the belt to take care of headspace. Several years ago I used a RCBS Precision Mic to measure new unfired Win. 300 Win. Mag cases. The average length to the shoulder was .006 shorter than fired cases. Using the false shoulder method to fireform all belted cartridges is the only way ;IMO; to insure that your cases fit the chamber with no stetching of the case above the web.
 
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