Case Length Shortening 2

[BOOM BOOM]

HI,
As some of you may remember I did a big experiment involving over 1000 rounds fired several summers ago.
Well I have fired another 250 squib loads this summer & had to trim again. For me the OAL does not shorten.
One variable I was not able to check (still can't) was a shortening OF THE LENGTH TO the datum line on the shoulder. Sure hope someone on the board could do that experiment.
SEVERAL good thoughts on causes were presented, along w/ a few short experiments.
I FELT MY EXPERIMENT WAS INCONCLUSIVE.
Well I just thought of another variable I did not take into account.
LOADING TECH.
I tend to load my bullets out to the point that clear resistance is met on closing the bolt of my 7MM/06. THIS WOULD PRECLUDE FIRING PIN STRIKE DRIVING THE CASE FORWARD.
THUS SETTING THE SHOULDER BACK SLIGHTLY.:roll:
This may be the reason I have never had a misfire on a squib load, while so many others have.:roll:

maybe a sticky on this issue ,w/ all the threads concerning it, would be usefull to some of the newer members of our board.

[405]

Not sure of the angle of your tests or the hypothesis but low pressure loads can cause a couple of things to happen or at least REVEAL themselves. Conversely, full pressure loads tend to mask the effects of headspace issues. One, as you mentioned, is the headspace created when the firing pin hits the primer. It is really obvious in those shouldered/rimless cases that have a small shoulder (small datum ring) with low angle (like the 17.5 degree shoulder angle cases). I'm convinced that not only the force of the firing pin fall can cause shoulder set back but also and maybe to a greater degree the detonation of the primer. Primer fires, primer moves rearward and case moves forward. When the case comes to full pressure the primer re-seats itself but at the expense of case stretching... the forward case body momentarily sticks to the chamber wall and the base and rear of the case is stretched and pushed rearward back into contact with the bolt face. Ultimately, this is a cause of case separation just forward of the web. For longer case life the idea is to minimize this condition by managing for minimum headspace.

Now the issue of case shortening or lengthening. In addition to the amount of headspace a firearm has, lengthening seems to depend on pressure and case design and possibly case hardness and/or alloy. I've noticed most shouldered cases fired at full pressure seem to lengthen over time. Some more than others. The only time I've seen shortening is in fireforming a parent case to the improved version. Then [my theory] to fill the extra room in the improved chamber in the region of the shoulder and upper case body, material is "borrowed" from somewhere. The material comes from the neck. The neck gets shorter so the case length gets shorter. Does this happens at all pressure levels... dunno?

You pointed out the best method of controlling headspace for any case that headspaces on the shoulder. Good method with bolt guns with some cam leverage. On new cases the best fit is when shoulder contact can be felt as the bolt drops to full closure. Probably squishing down the last couple thousandths at the datum ring on the shoulder. That assures best headspace fit. Then for subsequent reloads it's just a matter of keeping the sizing die away from the shoulder until the case stretches enough or fatigues enough to require setting the shoulder back a little. An alternate method is to seat hard cast bullets long enough so that they jam into the lands so the firing pin and primer detonation won't force the case away from the bolt face as the round fires. Obviously this alternate is not intended for firing jbullets at max loads!!

Of course fireformed brass, even using the above two methods, will never result in absolute zero headspace because of brass spring back- but it's as good as can be done and maintain reliable extraction and chambering. Headspacing issues and case life of full rimmed cases seem to depend more on the gun's inherent, mechanical headspace, but some control can be achieved in sprung guns and rimmed cartridges by seating cast bullets for jamming into the lands. This method does have its limitations though and can be problematic in lever guns.

[Murphy]

Boom Boom,

I'm not sure about a 7mm/06 (Wildcat round?) or if you meant 7mm/08. You may want to look into one of these. I've used one for years now and can't imagine loading without one.

http://www.midsouthshooterssupply.com/item.asp?sku=0004488322

Murphy

[mike in co]

HI,

One variable I was not able to check (still can't) was a shortening OF THE LENGTH TO the datum line on the shoulder. Sure hope someone on the board could do that experiment.
.


boom boom...simple tools exist for thei measurement. stoney point maks an entire set, or just buy the holder and the 30'06 shoulder gage.

i simple piece of metal with the correct dia hole, and a caliper will work. measure before and after, subtract the diff for an answer. remover primers prior to measure.

mike in co

and for 405..its headspace CLEARANCE, NOT HEADSPACE or change in case length as measured at the datum point.

[405]

and for 405..its headspace CLEARANCE, NOT HEADSPACE or change in case length as measured at the datum point.

Didn't think there was any confusion about terms. But whatever you
say :confused:

simple tools exist for thei measurement. stoney point maks an entire set, or just buy the holder and the 308 shoulder gage.

i simple piece of metal with the correct dia hole, and a caliper will work. measure before and after, subtract the diff for an answer. remover primers prior to measure.

All this assuming the gauge seats on the correct part of the shoulder (actually a datum ring NOT DATUM POINT) with no measurement errors. Those errors will likely mask length changes of .002" +/-. :coffee:

[mike in co]

and for 405..its headspace CLEARANCE, NOT HEADSPACE or change in case length as measured at the datum point.

Didn't think there was any confusion about terms. But whatever you
say :confused:

simple tools exist for thei measurement. stoney point maks an entire set, or just buy the holder and the 308 shoulder gage.

i simple piece of metal with the correct dia hole, and a caliper will work. measure before and after, subtract the diff for an answer. remover primers prior to measure.

All this assuming the gauge seats on the correct part of the shoulder (actually a datum ring NOT DATUM POINT) with no measurement errors. Those errors will likely mask length changes of .002" +/-. :coffee:

since we are looking for a difference in length, using a single tool with the correct dia hole is not likely to produce an error. the same user will normally produce repeatable results.
remember headspace for a given chamber can vary as much as 0.010 from the base number in saami and be in spec, it is not a single number for every chamber. again only looking for a diff in case length, nothing else.

proper terminology leads to more precise data exchange. propogating incorrect terminology is not good for precise discussions. its a simple thing.
mike in co

[joeb33050]

HI,
As some of you may remember I did a big experiment involving over 1000 rounds fired several summers ago.
Well I have fired another 250 squib loads this summer & had to trim again. For me the OAL does not shorten.
One variable I was not able to check (still can't) was a shortening OF THE LENGTH TO the datum line on the shoulder. Sure hope someone on the board could do that experiment.
SEVERAL good thoughts on causes were presented, along w/ a few short experiments.
I FELT MY EXPERIMENT WAS INCONCLUSIVE.
Well I just thought of another variable I did not take into account.
LOADING TECH.
I tend to load my bullets out to the point that clear resistance is met on closing the bolt of my 7MM/06. THIS WOULD PRECLUDE FIRING PIN STRIKE DRIVING THE CASE FORWARD.
THUS SETTING THE SHOULDER BACK SLIGHTLY.:roll:
This may be the reason I have never had a misfire on a squib load, while so many others have.:roll:maybe a sticky on this issue ,w/ all the threads concerning it, would be usefull to some of the newer members of our board.

We cover this topic in the book, Chapter 6.7 "ON HEADSPACE"
The Second Edition of the book "CAST BULLETS FOR BEGINNER AND EXPERT" is available at no charge on http://sports.groups.yahoo.com/group/CB-BOOK/, in "FILES".

Tom Gray has been able to iccrease the cartridge headspace and measure the increase(reduction of base-to-shoulder-datum distance) precisely, by having the firing pin hit the (dead) primer and move the case forward.

I have not yet been able to increase ctg. headspace by letting the firing pin hit the (dead)primer, and I've tried repeatedly.

I have been able to increase the ctg. headspace by letting the firing pin hit a succession of live primers in primed-only cases; and done this with three experiments now.

To keep squib = very low velocity loads from setting back the shoulder and increasing headspace on shoulder-headspacing cartridges, there are three methods that I've heard of:
1. Drill out the case flash holes. (#28 drill is frequently mentioned)
2. Leave the ctg. case/chamber slightly oily. (The instructions tell the user to lightly oil the chamber before firing the Competitor pistol.) John Bischoff is conducting an experimenjt about this.)
3. Make ctg. OAL such that the bullet/throat fit causes the ctg. to be sort of a "jam fit".

I had 30/06 cast bullet Unique slow loads increase headspace to the point of misfires, but that was years ago.

I know nothing about cases getting longer with squib loads, and would like to put any experimental results in the book. What say?

Mike will probably chime in with his incorrect "headspace definition" rant, just ignore him.

joe b.

[405]

What joeb33050 posted is pretty much as I have found.

BOOM BOOM, about measuring the headspace length on a case for precision and repeatability.... at least three gauges exist... the Stoney Point (now Hornady), the Redding with the dial indicator and the Wilson (a simple die form). I've found that the Wilson Cartridge Case Gage (cartridge specific) gives the most precise with the least margin for error. If I were setting up a test to compare measurements of the datum to base lengths for different loads, strings of loads, series of reloads, etc. I would use the Wilson. A direct, absolute measurement from base to datum is not needed- a baseline or index can be used with equal comparative precision.

First.... fire a factory, average bullet weight, full pressure Jbullet load in the test rifle. Measure using the gauge and calipers. That becomes the BASELINE, INDEX measurement for inherent headspace length in THAT rifle. By comparing that length to subsequent test cartridge headspace index lengths you can begin to develop data showing how different loads, pressures, reloading techniques, etc. affect case headspace length. With certain cartridges, as I stated in the first post, and at the lower pressure loads you may see actual shortening of the case headspace lengths. With those cartridges (shallow shoulder angles and small shouldered cartridges in particular) as the pressure of the test loads increase you'll likely see the headspace measurements increase and at some pressure the lengths will equal the baseline index length of the factory round. IIRC that happened around 30 thou PSI with most of the cases I tested a few moons ago. If you increase pressures much from factory full pressure any increase in measured case headspace index length will likely mean the action/receiver/bolt of the gun is beginning to give, spring or setback.... don't go there as that indicates excessive, damaging pressure!!!

Hope all this helps somehow

[BOOM BOOM]

HI,
The original experiment was titled. " CASE LENGTH SHORTENING MYTH OR REALITY?", IIRC.
The rifle is a 7MM/06 ,yes a wildcat, from which the 280/7mm express was developed for the masses.

Case length shortening has not been a problem for the rimmed cases or the belted cases to my knowledge. But I may be wrong , I just have never heard of it being a matter of concern in those type of rounds,due to the way they headspace.

The original thread brought forth several theroies as to why this case length occors.
Several board members conducted short experiments to support there ideas.

As I said , I have never had a misfire due to case length shortening, and ment to say it might be to my loading tech.
It was a variable I had NOT thought of as I did the original experiment.
The other variable I could not test, AT THE TIME, was a shortening at the datum line, rather than OAL.
This is why I considered the results still in question.

[jhalcott]

BOOM BOOM, that .280 (or what ever ) is a thoroughly enjoyable cartridge. I have a rifle in this caliber and a friend has it on the Encore frame with an 18" barrel!

[405]

BOOM BOOM,
Based on my experience, misfires due to such shortening would have to be really rare particularly in cartridges such as the 7-06. By wildcatting down from 30 cal the effect is a much larger shoulder surface area to prevent the firing pin/primer detonation from moving the case forward much. Regardless of the amount of shoulder or the basic cartridge design there is usually enough purchase by the extractor for the firing pin to detonate the primer.

As far as fact or fiction- it's a FACT! Given standard headspace (clearance)between chamber and case only those cartridges prone to the problem will show readily apparent signs of shortening between datum and base and only at the lower pressures. The "signature" of that condition is the same as excessive headspace--- the primers will be popped out slightly after firing. BUT the higher pressure loads in that same rifle and cartridge will show normal primer seating after firing. The higher pressure simply forces the case head rearward, stretching the case body and re-seats the primer. The examples I know of are the 06 wildcats, the Whelens, that maintain the 17.5 degree shoulder angle while reducing the amount of shoulder for the headspace control by necking up the 06 to the much larger calibers.

All one has to do to prove that type shortening is real not fiction is to take a 35 Whelen rifle with SAAMI headspace and use a case of normal SAAMI headspace (length datum to base) and fire some low pressure loads with something like 5744 and with the bullet seated short of the lands. EVERY one I have every fired showed at least some of the "signature" headspace problem.... primer slightly popped out after firing. When the basic inherent headspace of the rifle is on the "long" side of standard SAAMI specs the primers will be REALLY popped out. That's direct but qualitative evidence. Measure the datum to base length with a case gauge and that is quantitative data. Increase the load and pressure of that same combination and the readily apparent evidence and even the measurement evidence of the case gauge is hidden as the case is stretched rearward and the head is pushed back against the bolt face- re-seating the primer.