Brass actually SHRINKING when you fire it?

[Fu-man Shoe]

Greetings all,

Looking for some input here. Background is as follows:

I am taking a college course on statistical analysis, and as
a project for one of my assignments in said class, I am doing
an analysis on cartridge case OAL before and after firing.

I thought it was a cool kind of project. Involves data collection,
analysis, graphing, and best of all, spending some time down at
the range.

My hypothesis was that .38 Special ammunition, when fired from
a gun chambered in .357 Magnum will have a greater OAL than
the same ammunition fired in a gun chambered for .38 SPL,
due to the extra room in the .357 MAG allowing for greater expansion.

The results were interesting, to say the least.

It appears that the cases fired from the .357 MAG actually had
a *smaller* OAL than the cases fired in the plain ol .38 SPL.

This is contrary to what I had expected.

What do YOU think happened?

R,

fu-man shoe

 

[Nickotym]

The diameter of the chambers in each gun are probably not the same, thus affecting the OAL differently. You would have to have chambers almost exactly the same for this test.

 

[mete]

You have to use only one chamber for the experiment. On firing the case expands because of the pressure but in diameter to fill the difference between case and chamber.Little if any length change will occur. Most length change occurs due to alternating shooting and sizing when reloading, especially when using bottle neck cases and poorly adjusted dies. When I was reloading the 45acp I NEVER trimmed my cases ! ..So it looks like your initial premise was incorrect !

 

[mpmarty]

No, you are correct..

Low pressure loads have proven to actually shorten brass overall length. Reasons were put forth as pressure of load insufficient to totally obturate the brass to the chamber sides and resulting pressure curve as bullet went down barrel imparted to face of case caused shortening. Obversly, high pressure loads are thought to "liquify" the brass and cause it to "flow" in the only direction available to it, down the chamber making the brass grow in length.

 

[Steve in PA]

The chambers are the same, the only difference is in the length of the brass, .357 being longer than the .38's.

The cartridge headspaces on the rim of the cartridge so there should be no growth of the brass. There is no forward limitation to retard or prevent brass growth.

When the round is fired the pressure will drive the bullet forward, either opening the crimp, or leaving the mouth of the brass if there is no crimp.

Are you measuring the OAL on crimped or non-crimped brass? Are you using the same exact cartridge in the .38 and .357 chambers?

There may be a slight growth in one gun compared to another due to manufacturing tolerances. One gun may have a slightly great case head to frame difference than the other handgun. Have you taken this into account?

 

[ribbonstone]

Are you are shooting .38specials running at about 18K pressure vs. .357's running at something close to 30-35K pressure (assuming factory loads in both cases)?

 

[Jim K]

To run that experiment, you need to reload. Measure each case before seating the bullet and use a taper crimp, so seating doesn't crimp the case mouth or push the brass back. After firing, measure again. If you try to use factory ammo, you will have measurement problems.

FWIW, I think there will be no difference. .38 Special chambers have the shoulder far enough forward that for your purposes it will be the same as the .357 chamber. In other words, there will be nothing to stop the case from lengthening if it were to do so. But in a handgun, and especially the low pressure .38, I don't think there will be enough stretch to be significant or even measureable.

Jim

 

[Ol` Joe]

Fireing expands the case outward, filling the chamber. The brass to allow this has to come from somewhere and it causes the case to "shorten" somewhat. 45acp users see this all the time. The chamber of a semi auto is a bit larger then one for a revolver or bolt rifle. The cases will decrease in lenght measurably dureing the inital fireing. Later use of the same case won`t show as much reduction as the resizing never gets a case back to its original size. Measure a case before and after fireing and you will see this. Size the case with a resizing die after fireing and you will normally see the case lenghten somewhat. The amount depends on how much brass was initially displaced and how far back to the original geometery the die will rework the case.

 

[Fu-man Shoe]

I should make it crystal clear that this experiment has already been
run, and the results are in. As I posted, the .38 SPL ammo fired
in the .357 MAG pistol had a shorter OAL than the same factory ammo
fired in the pistol chambered for .38 SPL.

It would appear that some of you neither read nor understand the
question in my post before chipping in your two cents.

I appreciate the input, but only RELEVANT data is helpful.

Fortunately, a few erudite scholars out there actually READ my post.

Fireing expands the case outward, filling the chamber. The brass to
allow this has to come from somewhere and it causes the case to "shorten"
somewhat. 45acp users see this all the time.

Thank you 'Ol Joe.

Low pressure loads have proven to actually shorten brass overall
length. Reasons were put forth as pressure of load insufficient to totally
obturate the brass to the chamber sides and resulting pressure curve as
bullet went down barrel imparted to face of case caused shortening.
Obversly, high pressure loads are thought to "liquify" the brass and cause it
to "flow" in the only direction available to it, down the chamber making the
brass grow in length.

Thank you mpmarty.

Anyone else?

 

[Jeff F]

I'm thinking the .357 chambers are a little tighter then the .38 chambers. You would have to measure the diameter of the chambers.

 

[TX1911fan]

Fu-man, I was going to make the same comment you did about people actually reading your post before commenting. Cool experiment, interesting results.

 

[Dean C]

Would it be possible to make available your plan, protocol and results?
I too am having a bit of trouble with your conclusion.
Thanks,
dean

 

[Fu-man Shoe]

Certainly, I'm more than happy to share the results of my experiment.

Here is a copy of the assignment as I am turning it in tomorrow.

http://www.yourfilehost.com/media.php?cat=other&file=4965IND106_TSK6_THR.doc

(You have to download it from the provided link)

It is in .doc format with embedded .xls graphs. It opens up in
Microsoft Office just fine, but seems to have some trouble
displaying the graphs correctly within OpenOffice.

The meat of the experiment is right here though, in the graphed
data that I have collected. The first one here is the initial OAL
from both boxes of unfired cartridges.

The second one here is the graphed data that I collected AFTER
firing both boxes in a .38 SPL and a .357 MAG.

As you can see, both boxes decreased in OAL, but it appears that
the box of .38 SPL ammo fired in the pistol chambered for .357 MAG
actually shrunk more!

Interesting...

FYI, I used Remington UMC 130 gr. FMJ ammunition and a .357 Taurus
65 4'' barrel, and a .38 Taurus 85 2'' barrel. Firing was done at the
MMCS Camp Allen Marine Corps weapons range.

Fu-man Shoe

 

[Deavis]

Fu-man Shoe

No offense, but I think you are missing the point of many posters here. They are pointing out the flaws with your experiment, based on your hypothesis, in the first place. A flawed experiment gives you flawed data and any conclusion drawn from it is flawed. this is because you don't give the conditions of the experiment (at first), which any good scientific study would to answer the very questions those posters posed. It is useless to draw conclusions about a result if you don't know how the result was obtained. The listings in your report aren't very detailed to be honest, no offense, I just crunch DOEs everyday so...

Theoretically, the OAL you measure will be dependent on the pressure of the load and the chamber that was used. As pointed out, you should be using only one chamber, but since you didn't post any of the critical experiment conditions regarding your multiple chambers (exact diameter), the bullet weights (pressure), the charges in each (pressure), the OAL (pressure), etc from your factory boxes (you are just looking to justify your experiment which you believe to be untainted) we'll just assume that they are all perfectly matched.

For rounds that are below a pressure level X, they will not obturate fully in the cylinder. When the case comes out, you'll have a ice cream cone like effect of sorts with the flare beginning somewhere along the case length determined by the pressure. Since there is some displacement from parallel, this will show up in your OAL as shorter, if you can actually meaure precisely enough. Since you don't give your method or significant digits, we'll assume you are measuring on the order of .00005 minimum with a test fixture for any useful resolution at all.

Once you have reached pressure X, the case will completely obturate and any excess pressure can work to force the brass to lengthen. This is mostly seen in high pressure bottleneck cartridges where the brass can only flow forward after obturation. Unless you are above that threshold, your results will come back in the manner you describe, discounting the many things I listed above that could affect your study like crimp, pressure, measuring technique, etc...

Just out of curiosity, how did you measure the OAL of the case before firing if it had a bullet in it? If you didn't measure flush across the mouth at a minimum of 3 spots to find the true length, then your initial measurement is not comparable to the first measurement, ESPECIALLY, if the bullet has expanded the case or it has been roll crimped.

 

[Fu-man Shoe]

Deavis -

You bring up many good points. However, I believe you're overthinking it.

This is not a rigid scientific experiment that is going before a peer reviewed
journal. Nor is it some radical experiment whose purpose is to completely
revise the way we think about brass expansion in firearms.

Dude, it's a project for a 100 level college class. The other people in my
class? They're doing projects on things like statistical breakdown of how
many different colors come in a bag of M&M's.

Yeah...

So, while I agree that this is not a perfectly conducted experiment, according
to scientific protocall and everything like that, the point is that interestingly
enough, the data I collected goes against my initial supposition that the
cases would stretch, when in fact, they did the exact opposite.

Pretty interesting. A lot of people (formerly myself included) are under the
impression that pistol brass stretches. Turns out it doesn't.

/the more you know

Fu-Man Shoe

 

[Mr. Ouchie]

Hopefully he'll learn more by the time he finishes college...

 

[Fu-man Shoe]

Snide remarks were neither solicited nor appreciated.

Fu-Man Shoe

 

[Cellar Dweller]

This is gonna sound harsh, but I'm trying to be helpful

A lot of people (formerly myself included) are under the
impression that pistol brass stretches. Turns out it doesn't.

7.62x25 in a Tokarev or CZ-52 does stretch, because it's a bottleneck cartridge headspacing on the shoulder. Your conclusion is therefore false.

This is not a rigid scientific experiment that is going before a peer reviewed
journal. Nor is it some radical experiment whose purpose is to completely
revise the way we think about brass expansion in firearms.

Without controlling the chamber diameter or the ammo, you can't generalize that .38spl from a .38 shrinks more than .38spl in a .357 - you can conclude that in YOUR guns only.
Without rigid controls, your data collection is flawed; if the data is flawed, the conclusions from analysis are flawed (garbage in, garbage out). The lesson for your next project, free of charge, and worth exactly what you paid for it.

from your data pageAs the data collected by my experiment seems to bear out the above explanation, my conclusion is that the cases for pistol ammunition such as .38 SPL will not need to be resized or trimmed prior to reloading.

Perhaps unknowingly, you're doing "junk science." In this case, getting the right answer for the wrong reason. Would you get the same data if the .357 had larger chambers than it does?

 

[redneck2]

Hopefully he'll learn more by the time he finishes college...

Fu-Man Shoe says

Snide remarks were neither solicited nor appreciated.

Your replies to guys that try to point out errors in protocol and procedure seem to reflect an "attitude"

While many feel that this board would be frequented by toothless, pot-bellied pick-up driving hillbillies, you'll find that there are some extremely knowledgable and educated individuals. They've been through 100, 200, 300, and 400 level courses and beyond. Perhaps in their daily jobs, they're actually expected to obtain correct results.

Hopefully he'll learn more by the time he finishes college...

Doesn't just apply to what's taught in books.

An example....I was a Regional Service Manager for a very major corporation. We had some engineers that made what was thought to be a very questionable decision by some in our group. Their decision was correct (in their eyes), there's no way they could be wrong, and by god we're going ahead. Period. Ended up costing our company millions of dollars, thousands of man hours, and we lost a total market segment.

Now, your assignment may just be something to fill a paper for this week and your go on to something else next week. You're just fulfilling a requirement for the professor to check off the list. OTOH, at some time in the future, it may mean a LOT more.

HTH

 

[strambo]

I understand it is for a 100 level class. The point is with such minute changes being measured (case length in thousandths of an inch or less) any of the factors you did not control for could have had the same or more effect than the factor you were trying to observe (.357 vs .38 chamber length). As someone else pointed out...ramp the pressure way up and there would be room for expansion followed by stretching even in a 38 chamber, but the relatively low pressure of even 357 ammo does not usually result in case OAL increases. If a 357 won't stretch in a 357 chamber, a 38 never will (at safe pressures).

I would guess that the .38 average chamber diameter in that gun was a little smaller resulting in less brass expansion to the chamber walls. The 357 was on average a little looser in the chambers than that .38 resulting in more expansion to the chamber walls, hence the shorter OAL.

 

[strambo]

Hopefully he'll learn more by the time he finishes college...

I'll say it a lot nicer. College is about learning critical thinking skills IMO (how to think) not what to think. This is a problem with our colleges these days...critical thinking is going out the window (general statement, not aimed at you.)

So, who cares what your results were...the important point is what did you learn about the process? How do you view and understand interior ballistics now? How do you think about statistical analysis and the controls necessary to get meaningful data when observing minute changes? All the stuff Deavis was talking about. Leave the M&Ms to the underachievers...no reason not to do something well and correctly even if it is way more than required just for that class.

 

[Deavis]

I think the better conclusion would be:

"Based on this study's data and methods, there appears to be no increase in length brass."

Hey, I understand it is for a 100 level course, but it doesn't change the fact that you are making a blanket statement that is incorrect. Brass does/can stretch, it just depends on many factors not covered by your test setup. A bolt action rifle will show the most strech because the base of the brass is held firmly and the only place for brass to go, as it is being acted on by tens of thousands of PSI is forward.

Of course, an interesting thing to do would have been to resize the pistol brass, i.e. straightening the walls of each case. Then you would have a chance to actually remove the obturation and see if there was any stretch. That is why you resize before you trim anyhow. Furthermore, I was thinking a little about this, your pressure will also determine the rebound of the brass after it has obturated. For lower pressure you are well below the elastic limit for brass and probably expect no change at the accuracy that you are measuring.

Still, I'm curious, how did you get the initial OAL of the unfired case?

 

[Shear_stress]

This sounds like an interesting project. However, your methods are not rigorous enough to draw any hard and fast conclusions.

The term "average" has multiple meanings in statistics. Are you talking mean, median or mode? Second, a thorough statistical analysis would compare much more than averages: where are your standard deviations and p-values? Without these, there is no way to determine the consistency of your measurements and whether your results were simply due to random variation. The 0.003" difference in cartridge length you report between cartridges fired in .38SP and .357 wheelguns is within the 0.004" spread in the cartridge length before firing. Third, did you actually measure cartridge diameters pre-and post-firing, or just assume they were greater in the latter? Fourth, measurements in the range of thousanths of inches are approaching the limits of what most calipers are capable of. A slight change in finger pressure can change the measurment by a few thousanths. A micrometer would have been a better tool for this job.

I am not saying that you are wrong. You just need to provide more evidence to support your statements.

 

[Fu-man Shoe]

To all of you who gave intelligent and constructive feedback,
thank you very much. I have learned several things thus far,
one of them being that if you perform a scientific experiment,
it needs to be rigidly controlled, lest the nitpicker brigade
descend upon you, and pick apart your carefully constructed work.

But on the serious, I wish I could do a more in-depth study.
As it is however, I lack the resources and the setup to do a
scientific journal quality experiment. More's the pity, really.

However, for my purposes I am quite happy with the project.

I do have enough objectivity to agree however, that the results
and conclusion I arrived at are most likely true only for that gun
and that ammo. Truthfully, I hadn't considered the issue of
chamber size. I didn't consider that a factor, since chamber sizes
are all pretty standard within several thousandths of an inch.
(Obviously, or else they wouldn't fit the intended ammunition!)

Definately food for thought. Thanks everyone.

Fu-Man Shoe

 

[jonsidneyb]

This is what happens as some people have pointed out and there is a reason you can get different results if you change the variables.

1. When gun is fired expanding gasses expand and press the case against the chamber alls and the bullet is released in two ways. As the case expanded the crimp is relaxed but also the bullets forward motion pulls it free.

2. As the brass becomes stretch to a larger diameter and pushed against the chamber walls the length is pulled in slightly so at this moment it is shorter. Depending on the powder charge and yes and how much friction and weight there is to overcome pusing the bullet down the barrel, when either the bullet leaves the barrel or powder burn reaches a point that it cannot maintain the pressure the pressure drops.

3. If the pressure is sufficient at this point to compress the brass against the case walls the brass will start becoming thinner and longer, if the pressure is not high enough we should see a slightly shorter case depending on the difference beween the origianl diameter of the casing and the size of the chamber.

4. There is another variable in this mix. All brass strenght is not the same and the thickness is not all the same. That will also affect the results.

A weak mush load and a lose chamber would be more likely to have shorter brass come out then a super high pressure load in tight chambered example.