Experiment: How much does 5.56/223 brass grow in length?

My testing is intended to be specific to the AR15.

I always see the topic of trimming come up on message boards. Most of what I see posted is not based on actual observations with many varied opinions given. As a test to have some actual numbers about how often is trimming required I conducted an experiment. I also plan to conduct future experiments with variations.

Each test starts with a a brand new Lake City 09 case. In each test, the same case is reloaded and fired multiple times, measuring case length and other metrics of interest after each firing and each resizing. The specific reloading setup is listed in each test case below. Primers were reseated after firing so as to not affect the case length and shoulder bump measurements.

I was also curious about the neck sizing, and if work hardening would eventually result in needing a tighter bushing to achieve the same amount of neck tension. I intended to keep track of shoulder bump, but forgot the needed measuring tools at home. I don't know how much the shoulder was being bumped in Test 1, but will measure in subsequent tests.

Feel free to make observations and suggestions!

More Tests Completed. Each test started with a brand new LC 09 case.

Test 1: 20" Rifle Rainier, Initial Test. Expected to have least stretch because of no expander and rifle length gas and possible tighter chamber.
Test 2: 20" Rifle Rainier, Hotter load than Test 1, otherwise the same
Test 3: 20" Rifle Rainier, Hotter load than Test 2, otherwise the same
Test 4: 20" Rifle Rainier, Same load as Test 3, Switched to 0.244" bushing and installed expander button. Switched to +0.008" shell holder for less shoulder bump.
Test 5: 12" Carbine PSA FN, 0.244" bushing, Expander, and 0.008" holder. Same load and setup as Test 4.
Test 6: 12" Carbine PSA FN, Switched to 0.245" bushing, No Expander, and a +0.010" shell holder to reduce shoulder bump.
Test 7: 12" Carbine PSA FN, Same setup as 6, but with reduced load. Also annealed and trimmed once brass became over length.
Test 8: 16" Midlength
Test 9: 20" Rifle Lothar Walther, Not enough shoulder bump.
Test 10: 20" Rifle Lothar Walther, Repeat of Test 9 with 0.006" shell holder.

The hotter loads by themselves did not cause the cases to lengthen. I feel like I was close to the limit pressure wise. A 69 gr out of a 20" barrel at nearly 3000 fps is pretty decent. The primer pockets were getting pretty loose by the 5th or 6th firing.

With the expander button present and an extra thousandth smaller bushing (Test 4), the case did finally start to lengthen though it didn't ever make it past the maximum. I feel like the case was pretty much done by the end of Test 4.

The 12" carbine definitely stretches case. Test 7 should be as good as it gets for case stretch as it's a reduced load, no expander, and optimal shoulder bump. Tests 4 and 5 are directly comparable as they used the same load and setup.

So the results currently are
1) The 20" rifle has a lot less brass stretching than the 12" carbine, possibly not requiring trimming ever. The 12" carbine may require trimming in as little as one firing.
2) The expander button does appear to cause case length growth. The 12" carbine experience case length growth even with no expander button and minimized shoulder bump.

Reserved2

Would be curious to see the results if you did not resize and only deprimed. I remember reading an article saying it's the resizing that stretches it out. The article claimed that resizing takes it back to SAMI specs which your rifle may or may not be. It said that once a casing has been fire formed to the chamber, resizing is not necessary, assuming the chamber is not so far out of specs that the bullet wouldn't seat properly.

This was an AR15. Need to do some form of FL sizing for operation. I have heard that though, but it's always been in the context of a tight chambered benchrest type rifle.

Work hardening will eventually result in split necks. My experience is that split necks begin to show up at around 13 reloads. Accuracy goes south before that.

The overall results may vary with each individual gun because of varying actual chamber dimensions. Metal stretching and work hardening will depend on just how much the metal moves so a larger chamber will result in more deformation. Also different manufacturers cases differ in as manufactured dimensions such as wall thickness as well as internal grain structure which results from the processing steps and annealing schedule. Bending a paper clip back and forth half way will take longer to work harden than bending it fully.

Your experiments will be interesting but the actual results may be different in different firearms or with other manufacturers cases who were originally processed with different annealing schedules.

Another issue, a lot of stretching comes from the expander ball being pulled back through the neck.

So using a bushing die, you are reducing the over sizing/expanding of the neck.

That will also skew your results on work hardening.

Pinecone said:

Another issue, a lot of stretching comes from the expander ball being pulled back through the neck.

So using a bushing die, you are reducing the over sizing/expanding of the neck.

That will also skew your results on work hardening.

Click to expand...

Bingo...interesting though...
too many variables ...my head hurts..
gun specific..chamber specific...barrel specific..

have fun...

-Rock

Well, we already have one test with ZERO case stretch which in itself is interesting.

atblis said:

Well, we already have one test with ZERO case stretch which in itself is interesting.

Click to expand...

yes sir it is ...gonna keep on eye on this and see how it flows.

-Rock

Add gas system length to your list of variables. I suspect that matters. I have several different ARs to chose from, so I'll experiment. I do not have a 16" with carbine gas as I prefer mid lengths. I do have a 12" with carbine gas.

I seem to remember reading that brass changes with time. If you re-size and then let cases sit, they'll have different properties compared to those that were freshly re-sized. I seem to remember reading about this in the context of neck tension and bench-rest stuff. Any one know what this phenomenon/property might be called if it does exist?

Pretty sure a ton of growth also comes with higher pressures as well..... I batch process on a 1050 but the only thing I can contribute is known,once fired, 556 usually I hear the trimmer working longer than my mixed range practice rounds. Occasionally with my 75 gr round the trimmer barely touches the case as I cycle the press sometimes. All I got on this.........

atblis said:

Add gas system length to your list of variables. I suspect that matters. I have several different ARs to chose from, so I'll experiment. I do not have a 16" with carbine gas as I prefer mid lengths. I do have a 12" with carbine gas.

I seem to remember reading that brass changes with time. If you re-size and then let cases sit, they'll have different properties compared to those that were freshly re-sized. I seem to remember reading about this in the context of neck tension and bench-rest stuff. Any one know what this phenomenon/property might be called if it does exist?

Click to expand...

Certain metals and alloys "age harden" actually it is Precipitation Hardening and is common in some aluminum alloys but also can happen with some copper alloys including brass and in other metals. Google, age hardening of brass, for lots of reading.

More measurements added. See Post 2. The comments will change. Just my initial thoughts.

Can you add pre-fired data (length, diameter....)?

I think what you're asking for is in there. The "initial" row is the case length before any firing. It kind of goes in order from left to right.

Excellent experiment!

I would suggest that the amount of your shoulder displacement during sizing is actually what will grow cases by a measurable and predictable amount and would like to see that data added to your list of variables.

Most people take a too aggressive approach to full length sizing. Usually, the handloader follows the die maker's instructions, which are intended to be a foolproof, one resizer fits all solution. This chronic maladjustment for 95% of rifles out there is one reason many people operate under the false impression that sizing only the neck will extend the life of a cartridge case. It will not appreciably extend the life of a properly full length sized case, IF the full length is correctly set for your chamber.

If your chamber is short (minimum headspace), then setting the sizer all the way down as per instructions isn't too destructive and you don't see a lot of premature failures related to stretching, nor will trimming seem excessive. If one loads for multiple rifles, they may be forced to take this approach, but it is best avoided.

Problem is that most rifles in general, and almost every factory rifle I've ever seen, is nowhere near minimum. Most factory rifles would actually qualify as having somewhat large chambers, in order that every factory rifle swallow every factory load out there.

The difference between minimum and maximum headspace dimensions in the .223 is .007". If you full length size down to minimum and your rifle runs a little long, say .005", then you are inducing approximately that much stretch during each firing, a goodly portion of which will end up as shavings under your trimmer.


The mechanism at work is as follows:

1) We induce a gap between case shoulder and chamber by whatever amount we overdo the full length sizing.

2) When the cartridge is chambered, it only moves as far forward as the bolt face/ejector spring can push it.

3) Upon firing, the firing pin impacts the primer and the case is moved forward as far as it will go. When this happens, the gap at the shoulder is closed and become a gap between the bolt face and case head. This gap is called headspace and will measure the same as whatever dimension we pushed the shoulder back compared to the actual chamber. Since we just returned the case to minimum and we have a chamber .005" longer than minimum (still .002" under max headspace), then our cartridge headspace is .005".

4) Once the case has gone up against the chamber shoulder, the firing pin can indent the primer enough to cause ignition.

5) The increase in pressure during powder combustion is a relatively gradual process, with pressures rising quickly to peak, then dropping off as the powder is consumed and the bullet moves out.

6) The case walls are tapered, becoming quite thick toward the head. As pressure builds, the thinner parts of the case yield first. This means the case neck obturates to full diameter as the bullet is released.

7) Pressures continue to build as the charge burns and the bullet engraves into the rifling.

8) The shoulder is already in hard contact with the chamber, driven to that point by the firing pin spring. As pressures build, the top portion of the case body expands and grips the chamber, and this effect continues down the ever-thickening case walls.

This is the key concept which makes it all make sense: The expanding case tightly grips the chamber while pressures are still at a working level. This grip cannot be broken, In fact, the case head will tear off (separate) before the body lets go of the chamber walls.

9) As pressures increase, the gradually thickening case walls increasingly expand, until we are at the "web" of the case, where this relatively flexible case body meets the relatively inflexible case head. As the case walls expand, the case becomes locked into the chamber.

10) We reach the point in the pressure curve in which the case has fully expanded in the chamber, yet there remains a .005" void between the case head and bolt face. While the case is locked in place by pressure, the case head, the final portion of the case to move, is slammed back against the bolt face, closing the gap, stretching the case to fit.

11) This stretch occurs right at the junction of the case body and case head, in the same old spot, characteristic of case head separation. At this point, the case is approximately .005" longer that it was after being forced into the die.

12) Provided we did not encroach upon the cartridge case's limit of elasticity, as pressures subside, the case body springs back away from the chamber walls, releasing its grip. If we did happen to run pressures too high, we exceed the limit of elasticity and the case fails to spring back as pressures subside. This is what leads to sticky extraction in high pressure loads.

13) Now, we full length resize the case for the next loading. The shoulder position is restored to minimum, where we left the die set according to manufacturer's directions, but the neck is longer, and will need trimming much sooner that cases processed on correctly adjusted equipment.

The problem is that we cannot put brass back where the case has been stretched and thinned. Brass that has migrated forward cannot be put back.

As we resize the case, it doesn't get pushed back shorter, it just slides into the die and reforms the features in their new location. While some of the growth is concealed as we move the shoulder back, we will still see cases stretch at greater than necessary rates. Inducing this stretch by a maladjusted sizing die will lead to additional trimming.


To help negate this effect, I try to set my full length sizer to set the shoulder back just far enough to ensure clean function. With bolt guns, this can be .001" to .0015", gas guns tend to be more reliable with .002" or so.

While following the manufacturer's instructions is usually the best way to proceed, in this case we must recognize the die maker's goal of your loaded cartridge fitting ANY standard chamber out there far outweighs his concern for your case life. We do not share priorities here, and for best results, full length dies should be set to the individual rifle. If we take a one size fits all approach and blindly follow instructions, we can expect our reloads to fit any gun, but we should also anticipate extra trimming and a reduction in case life.

What is your hypothesis?

I have several theories I wanted to test:

1) If you trim sufficiently short initally, you will not need to trim a case again.
2) If you do not use an expander button, the case length growth will be reduced.
3) Resizing using optimized shoulder bump and neck bushing will reduce case length growth.
4) Shorter gas system barrels cause greater case length growth

Shoulder displacement is a variable I am measuring. I have used the term "shoulder bump" for the difference between the fired and the resized shoulder location. The measurement was taken using the Hornday comparator attachment on calipers with the 0.330" bushing.

If you take a look at Test 4, notice that I went to a +0.008" shell holder to reduce the shoulder bump to an appropriate amount. Also consider that in the Test 1, 2, and 3 the shoulder was being set back what I would call excessively. In fact, I did check a resized round in a Dillon 223 case gauge, it was a touch below the SAAMI minimum step.

I think my next test will be to use the resizing setup of Test 4 with the load used in Test 1. I will then test the 12" carbine gas pistol upper I have.

For those unfamiliar with the Redding competition shell holders check out this link.
http://www.redding-reloading.com/online-catalog/35-competition-shellholder-sets

I may have missed something, but shouldn't the base line of these tests begin with new, factory loaded rounds measured pre and post firing? If the cases stretches the stretched portion of the brass would need to come from the neck wall thinning? Once fired would have already stretched and thinned resulting in skewed data?