7MM Rem Mag Brass Stretch

R

Renegade19sc

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Is it normal for 7MM Rem Mag brass to stretch a lot on the first firing of a factory case. I know the headspace is measured in the belt but is there a range measurement from the datum line to the head of the case like a rimless case (Example 30-06, 308, etc). I have some cases that were fired from my gun and compared them to factory brass of the same type that has not been fired and the stretch is roughly 8 to 10 thousands by using a RCBS Mic.
 
It's normal. As you found out, the brass stretched to fit your chamber's dimensions. Now, to avoid unneeded brass fatigue, simply neck size your brass to avoid shoulder setback and unwanted headspace.

The brass cannot get any longer/bigger than the dimensions of the chamber in which it is fired. It WILL lengthen in the neck from internal brass flow...which is normal in all cases at the upper pressure limits. Moderate/midrange loads will greatly limit case stretch, and more loadings before trimming is needed. When you feel a little [snugness, or, friction/resistance] upon bolt closure...THEN....set the shoulder back a thousandth or two and brass life will be extended as well as chambering will be easier. Just be sure to adhere to case length specs.

I've gotten as many as 12-15 loadings from the brass in my Ruger 77 7 Mag.

WILL.
BTW: when barrels are chambered, the reamers are purposely ground oversized to allow for wear on the reamer during each chambering, purely for economic reasons. This allows the manufacturers to keep costs down by not having to use so many reamers. Anywhere from 25-30 or so barrels can be chambered with the same reamer before regrinding (recycling) the reamer to a smaller dimensioned cartridge, again....cutting costs by not having to start from scratch making new reamers.

Ultimately... each successive chambering following the initial (largest/loosest), will then be smaller/tighter until the VERY MINIMUM SAAMI specs are met and then they start over with a new reamer.

The result is: What you experienced. That is why .....A fired case from a rifle chambered toward the end of a reamer's "run" (smaller/tighter dimensions) will chamber in a rifle from the reamer's initial few chamberings (larger/looser), but the converse (reverse) is not true: [ First-reamed-chamber fired case will not/cannot be fired in the lastly reamed chambers].

Now....For the ammo: Factories are aware of this practice, so they manufacture ammo to allow the cartridges to chamber in the rifles which were reamed toward the end of the reamer's cycle, (minimum SAAMI specs). SO... Case stretch such as you experienced...And... The CYCLE continues.
 
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Also the bulge in front of the belt will have to be addressed at some point.
I full length size my 7mm rem mag brass everytime. And some of my brass has been loaded and fired 12 times, and looks fine inside and out....I do not load max, I load for precision...

Forster benchrest dies BTW.
I bought the Larry Willis die that cures the bulge, turns out it was not necessary.
 
Manufacturers at one time became very sloppy about dimensions --"you headspace on the belt so shoulder dimensions aren't important " :rolleyes:
That meant head to shoulder dimensions were too long. Sizing a case should always be to the chamber , not to standard case dimensions !! :)
 
Keep in mind that most rifle makers don't give a hoot about reloading(even forbid using reloaded ammo) as long as the case doesn't blow out on the initial firing, it's just fine.
 
For the fans of belted cases, which I'm not, you have to realize that the brass will have tolerances as to the belt distance from the case head. Then the chambers have tolerances in manufacturing. With a little bad luck, you can end up with a short brass measurement, used in a long chamber. Put a round in the chamber, close the bolt, and shake the rifle back and forth: you'll hear the round thumping as it rattles around in your 'precision' rig! Once you fire new brass, you can eliminate most of this by controlling your shoulder setback, and making it headspace on the shoulder like non-belted cases.
 
Is it normal for 7MM Rem Mag brass to stretch a lot on the first firing of a factory case
Click to expand...

If there was any truth to "the firing pin strikes the primer and drives the case forward until the shoulder of the case collides with the shoulder of the chamber etc.", there is nothing you can do about it because the case stretches 'EVERY TIME!:eek:.

F. Guffey
 
The 7mmRemMag starts out headspacing on the belt. The shoulders are pushed back on factory ammo and new factory brass.
The SAAMI spec on 7mmRemMag brass is 0.220 - 0.008".
The SAAMI spec on 7mmRemMag brass is 0.220 + 0.007".

I have not found any 7mmRemMag brass bigger than 0.215", so I chamber my 7mmRemMag rifles at 0.215".
I don't do anything about my factory 7mmRemMag rifles.
The cases are going to get stretched on the first firing, but a less in the rifles I chambered.

I try to push the shoulder back ~~ 0.001" more than needed for chambering, when resizing. Thereafter the ammo is case spacing on the shoulder.

[On the first firing the belt stopped the forward push by the firing pin. On the second firing the shoulder stopped the forward push by the firing pin.]


I have fired 7mmRemMag brass at different pressure levels:
1) So low, the reloads will fit in any 7mmRemMag
2) A little higher and Larry Willis' die will be needed to make the brass fit in other rifles that are tighter diameter in front of the belt.
3) So high Larry's collet in front of the belt will not make the brass fit in tighter diameter chambers.
4) So high the extractor groove expands, but the primer pocket still feels tight when inserting a new primer
5) So high the primer pocket feels loose.
6) So high the new primer falls out of the pocket
7) So high the primer pocket gets huge and extraction requires pounding.
8) So high the bolt deforms behind the lugs and the receiver abutments deform making the bolt very hard to get out or back in the receiver.

I buy once fired from a test lab and try to load between #3 and #4 above.
I never use the brass again after I fired it. I could use it, if I could keep used brass segregated by which rifle fired it. The only thing that introduces more entropy into my kit than hunting, is going to the range. I can't seem to segregate fired brass.
 
Yes stretch to the datum line will be common with a round that headspaces on the belt. Honestly even a standard cartridge would likely have a measurable stretch (unless you headspaced your rifle to fire a specific brand of factory ammo like I did).

So it's normal. Resize with a collet (neck only) die, or set up a full length die to just bump the shoulder back enough to be a snug (but not tight) fit in your chamber. I used to neck size only, but after having to manhandle by bolt handle after the fourth firing, I prefer to full length size after 3 firings or so. I shoot hot loads (still within published max loads though), so I have primer pocket issues long before the neck is in danger of splitting from being worked too much by full length sizing.
 
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I have some cases that were fired from my gun and compared them to factory brass of the same type that has not been fired and the stretch is roughly 8 to 10 thousands by using a RCBS Mic.
Click to expand...

"I have some cases that were fired from my gun", and I wonder, why didn't you measure the factory over the counter new ammo length before firing instead of after.

Again, I have a M1917 with a chamber that is .016" longer from the shoulder to the bolt face than a minimum length/full length sized case. I measured the length of the chamber with a 280 Remington case. The first time I fired the rifle with that long chamber I had .002" clearance between the case head and bolt face.

Forgot, the .002" clearance is the magic amount of bump? when sizing a case.
I could say "I bumped the shoulder .014" " but in my mind what I did to the 280 Remington case should be described as 'near wreck' not bump.

F. Guffey
 
If one expands the neck with a mandrel, and the then pushes the tiny shoulder around the neck back to zero case space, there will be no stretch on the first firing.

The late gunsmith Randy Ketchum had a 243 that had a thread too much headspace. He never bothered to fix it, he just made the brass longer before the first firing.
 
The late gunsmith Randy Ketchum had a 243 that had a thread too much headspace. He never bothered to fix it, he just made the brass longer before the first firing.
Click to expand...

What receiver was Randy using, I did not know him but I believe I could have helped him with his problem if he thought the firing pin drove the case forward until the shoulder of the case collided with the shoulder of the chamber.

I have fired cases with .127" clearance between the case shoulder and chamber shoulder.

F, Guffey
 
I have fired cases with .127" clearance between the case shoulder and chamber shoulder.
Click to expand...

And that is the reason I said it is assumed the case stretches and there is nothing a reloader can do about it. For most that is a sounds like a conflict.

F. Guffey
 
There are probably other ways to do it.
My late father referred to Molybdenum grease as 50,000 psi grease.
I am sure a shoulder could form and a case could slip back to the breech face at less than 50kpsi.
It's messy.
 
In loading for several 7mmRM over 40 years, the one constant mentioned is true - I don't FL size unless using new brass. FWIW, I've not had to FL size new brass, either. Eezee peezy, japaneeze.
 
I am sure a shoulder could form and a case could slip back to the breech face at less than 50kpsi.
It's messy.
Click to expand...

In the big inning they did not care, they had a rim and they had a belt. What ever happened in front of the rim and or belt happened. That was before the famous quote: "The firing pin drives the case shoulder to the chamber shoulder and then the firing pin crushes the primer". Then there was Hatcher.

I am not a slide and glide shooter. I form cases to fit, then fire. Others fire to form. And I am always curious, they always omit the make and model of the rifle as thought that makes little to no difference.

F. Guffey
 
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