ABOUT CARTRIDGE CASES

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  • Last Post 27 December 2012
joeb33050 posted this 25 December 2012

ABOUT CARTRIDGE CASES

This summarizes the results of tests and measurements performed in 2012. 

For detailed test results and measurements see 6.2 CASE SHOULDER SETBACK WITH REDUCED LOADS and/or 6.2 CARTRIDGE HEADSPACE AND LENGTH CHANGES in CAST BULLETS FOR BEGINNER AND EXPERT, THIRD EDITION.

Definitions of these words as used below are from the SAAMI glossary: HEAD CLEARANCE The distance between the head of a fully seated cartridge or shell and the face of the breech bolt when the action is in the closed position. Commonly confused with headspace. HEADSPACE The distance from the face of the closed breech of a firearm to the surface in the chamber on which the cartridge case seats.

Thanks to Kent Sakamoto at ATK/RCBS for lending me the RCBS “PRECISION MIC” headspace measuring micrometers used in these experiments.

CARTRIDGE CASES DO GET SHORTER WHEN THEY ARE FIRED; AND LONGER WHEN THEY ARE FULL LENGTH SIZED. As much as .007” for .223 Rem. and .243 Win. cases, and as much as .013” for .308 Win cases.

PULLING THE CARTRIDGE CASE OVER THE EXPANDER BUTTON IS NOT THE CAUSE OF CASES GETTING LONGER WHEN THEY ARE FULL LENGTH SIZED.

CASE HEADSPACE IS REDUCED AS FIRED CARTRIDGE CASES ARE FULL LENGTH SIZED. As much as .008” depending on chamber headspace and sizing die adjustment.

SIZED CARTRIDGE CASE LENGTH VARIES SLIGHTLY AS THE FULL LENGTH DIE SETTING VARIES. Die setting instructions for FL sizing vary by manufacturer and in the literature. Maximum of .002” for .223 Rem. and .308 Win. cases.

CASE HEADSPACE VARIES AS THE FULL LENGTH SIZING DIE SETTING VARIES. As much as .004” in .223 Rem., and .008” in .308 Win.

NECK SIZING IN A FULL LENGTH SIZING DIE DOES NOT CHANGE CASE LENGTH.

NECK SIZING IN A FULL LENGTH SIZING DIE INCREASES CASE HEADSPACE. Depending on die setting etc., as much as .005” in .308 Win. cases. Could this have to do with “bumping back” case shoulders?

EXPANDING/BELLING CASE NECKS WITH THE LYMAN “M” DIE DOES NOT CHANGE CASE LENGTH OR CASE HEADSPACE.

SIZING IN THE LEE COLLET DIE DOES NOT CHANGE CASE LENGTH OR CASE HEADSPACE.

REPEATEDLY SIZING IN THE LEE COLLET DIE DOES NOT CHANGE CASE HEADSPACE.

NECK SIZING IN A LEE LOADER DOES NOT CHANGE CASE HEADSPACE.

THE PUSH FEED EXTRACTOR DOES NOT CHANGE CASE HEADSPACE.

FIRING VERY LOW VELOCITY LOADS, (VLVL), DOES SET THE SHOULDER BACK, DECREASING HEADSPACE.

SHOULDER SET BACK WITH VLVL IS CAUSED BY THE PRIMER FIRING.

SHOULDER SET BACK WITH VLVL IS NOT CAUSED BY THE FIRING PIN HITTING THE PRIMER.

SHOULDER SETBACK WITH VLVL IS NOT CAUSED BY THE PUSH-FEED EXTRACTOR.

THE EXTRACTOR DOES LIMIT SHOULDER SET BACK WITH VLVL.

SHOULDER SETBACK WITH VLVL CAN BE ELIMINATED BY OILING THE CASES BEFORE FIRING.

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frnkeore posted this 25 December 2012

good article, joe.

frank

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delmarskid1 posted this 26 December 2012

When cartridges get longer from full length sizing does the entire case grow or just some portions such as the neck, shoulder or body?

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joeb33050 posted this 26 December 2012

delmarskid1 wrote: When cartridges get longer from full length sizing does the entire case grow or just some portions such as the neck, shoulder or body? I don't know. FL size-cases get longer AND case headspace-from shoulder to base, gets shorter. But, as the fl die is adjusted downward, case length stays the same, then gets LONGER as neck is sized, then gets shorter as the die gets to the shell holder. The data shows this clearly. I don't know why or what happens. joe b.

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TRKakaCatWhisperer posted this 26 December 2012

When one fires the ctg, the case is pushed forward, the primer goes off, the front part of the case seals tightly. (How tightly? Any slip? - it's a variable.) The back part of the case will often be stretched back to fill the clearance between the base and the bolt face. This will stretch the case about 1/2” above the base. Evidence is the appearance of a stretched-thin area which will turn into a case separation and the base falls off. (It can be discovered by taking a paper clip - bending an 1/8” part at the end 90 deg. and feeling INSIDE the case.) It can often also be seen as a shiney ring outside the case just above the base.

THEN, full-length sizing moves the shoulder back, the neck becoming a little longer.

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joeb33050 posted this 26 December 2012

Then the case gets longer as it is fired over and over, and what gets longer is the neck. My CB cases don't seem to get longer. Maybe I can test/experiment and measure this. I have to think about it. joe b.

TRK wrote: When one fires the ctg, the case is pushed forward, the primer goes off, the front part of the case seals tightly. (How tightly? Any slip? - it's a variable.) The back part of the case will often be stretched back to fill the clearance between the base and the bolt face. This will stretch the case about 1/2” above the base. Evidence is the appearance of a stretched-thin area which will turn into a case separation and the base falls off. (It can be discovered by taking a paper clip - bending an 1/8” part at the end 90 deg. and feeling INSIDE the case.) It can often also be seen as a shiney ring outside the case just above the base.

THEN, full-length sizing moves the shoulder back, the neck becoming a little longer.

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John Alexander posted this 26 December 2012

TRK writes:

"When one fires the ctg, the case is pushed forward, the primer goes off, the front part of the case seals tightly. (How tightly? Any slip? - it's a variable.) The back part of the case will often be stretched back to fill the clearance between the base and the bolt face. This will stretch the case about 1/2” above the base. Evidence is the appearance of a stretched-thin area which will turn into a case separation and the base falls off."

That is exactly right and also the reason you should throw away the instructions that come with full length dies to run the die down against the shell holder UNLESS YOU ARE LOADING HUNTING OR DEFENSIVE AMMUNITION where a bit of dirt might prevent chambering a round when it makes a big difference.

For casual or target shooting the possibility of head separation can be reduced in many calibers (and avoided in others) by adjusting the FL die down a bit at a time while trying the case in the rifle. When the case will chamber with just the slightest “feel” on the bolt stop turning the die down. Thus there will be little or none of the clearance between bolt face and rear of case mentioned by TRK except that caused by the primer ignition (in some calibers.)

If the caliber/primer combination is one where the case is driven forward by the primer as documented by Joe B. with the 308 (thus creating some clearance) there will still be some stretching unless you oil the case. But not as much as if you start with a case fully sized down (with the one size fits all instructions) to usually quite a bit less case headspace than needed for the individual rifle . With some caliber/primer combinations (223 for one) the case isn't shortened by the primer ignition, and for these case separation should be avoided completely if you only FL size just enough to chamber easily. Of course only neck sizing with a neck size only die such as the Lee collet die or Lee loader (not a FL die backed off) will achieve the same minimum clearance.

Following the manufacturer's recommendations will almost always work but sometimes it isn't the best for an individual rifle.

John

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LWesthoff posted this 26 December 2012

Afraid I'm a little bit confused. If “headspace” is “the distance from the face of the closed breech (bolt face?).....to the surface in the chamber on which the cartridge seats (some point on the chamber shoulder?)” then it looks to me like the only way you could “increase the headspace” is by mechanically changing the chamber dimensions. Am I misreading something, or is something wrong with the stated definition of Headspace?

Wes

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joeb33050 posted this 26 December 2012

There's “chamber” headspace, and “cartridge case” headspace. Both ~ the same dimension. The definitions given are SAAMI, who don't define cartridge case headspace, but do show the dimensions of it on their drawings. joe b.

LWesthoff wrote: Afraid I'm a little bit confused. If “headspace” is “the distance from the face of the closed breech (bolt face?).....to the surface in the chamber on which the cartridge seats (some point on the chamber shoulder?)” then it looks to me like the only way you could “increase the headspace” is by mechanically changing the chamber dimensions. Am I misreading something, or is something wrong with the stated definition of Headspace?

Wes

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TRKakaCatWhisperer posted this 27 December 2012

LWesthoff wrote: Afraid I'm a little bit confused. If “headspace” is “the distance from the face of the closed breech (bolt face?).....to the surface in the chamber on which the cartridge seats (some point on the chamber shoulder?)” then it looks to me like the only way you could “increase the headspace” is by mechanically changing the chamber dimensions. Am I misreading something, or is something wrong with the stated definition of Headspace?

Wes

That's why I chose to phrase it: clearance between ctg head and bolt-face. The terms have changed meanings over time. JoeB has some good illustrations with the terms on them. Yes, changing the 'headspace' (all definitions) takes mechanical adjustments. A gunsmith uses go / no-go gauges to check it. In some machineguns (M60 and M2 for example) it is adjusted and set with a head-space and timing gauges.

But YOU can adjust how much clearance there is in your 'iron by how much you full-length size it (as John A. delimited). Some clearance is needed. A lot of clearance is not cool. How much depends on the application.

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LWesthoff posted this 27 December 2012

Actually, with years of handloading both jacketed and cast bullets, I quite agreed with Joe B's original post. The only part I stumbled over was SAAMI's definition of “headspace” and subsequent references to “case headspace.” Just didn't catch the difference.

Wes

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TRKakaCatWhisperer posted this 27 December 2012

LWesthoff wrote: Actually, with years of handloading both jacketed and cast bullets, I quite agreed with Joe B's original post. The only part I stumbled over was SAAMI's definition of “headspace” and subsequent references to “case headspace.” Just didn't catch the difference.

Wes

Yup. JoeB has a GOOD illustration with the terms.

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joeb33050 posted this 27 December 2012

I get a set of 120 or so cases for a gun, fire, size, trim and load. Then neck size. When a case neck splits, I anneal the lot, wash, neck size, load. My cases last almost forever, losing one now and then to neck splits. I've never had cases lengthen, that I can recall. The original trim is so the ” die works the same on all. I've never had a case separate, that I can recall. In the cast bullet context, I'd like to know if anyone has had cases either lengthen or separate. I don't know how to test for this,I think that's what I'm doing when I shoot. I think for the TRK thing to happen, that there must be a set of circumstances not seen in CB shooting. N'est ce pas? joe b.

joeb33050 wrote: Then the case gets longer as it is fired over and over, and what gets longer is the neck. My CB cases don't seem to get longer. Maybe I can test/experiment and measure this. I have to think about it. joe b.

TRK wrote: When one fires the ctg, the case is pushed forward, the primer goes off, the front part of the case seals tightly. (How tightly? Any slip? - it's a variable.) The back part of the case will often be stretched back to fill the clearance between the base and the bolt face. This will stretch the case about 1/2” above the base. Evidence is the appearance of a stretched-thin area which will turn into a case separation and the base falls off. (It can be discovered by taking a paper clip - bending an 1/8” part at the end 90 deg. and feeling INSIDE the case.) It can often also be seen as a shiney ring outside the case just above the base.

THEN, full-length sizing moves the shoulder back, the neck becoming a little longer.

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TRKakaCatWhisperer posted this 27 December 2012

With cast bullet pressures I've experienced LITTLE case lengthening.

With higher velocity/pressure bullets it's another story. Sometimes 5 firings between trimmings.

Shallow shoulder angles (.30-06 and such) lengthen more easily. Steeper angles (40 degrees vs 17 degrees) experience MUCH less stretching.

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John Alexander posted this 27 December 2012

My experience matches TRK's.

I get some lengthening with my 270 which mostly sees only full charge loads.

My match 223 cases are made from 22 mag cases and fitted to almost eliminate the gap at the end of the throat (.003” or so). As a result I watch then very carefully. They now have been reloaded 35 to 40 times all low pressure loads and only neck sized with a collet die. I have trimmed a couple of thousandths on a few of them but am not sure that any of them have lengthened as opposed to me seeing minor differences in my measuring over time. Most haven't been trimmed at all.

I also have never had a case head separation with either jacketed or cast.

John

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