The curious case of the compressed powder charge


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Seismic Sam
March 9, 2010, 07:24 PM
To begin with, please refer to this thread as a preface to what I'm about to say:

http://www.thehighroad.org/showthread.php?t=502850&page=2

While I was loading up a bunch of 357 SIG brass using 125 grain Speer Gold Dots with AA#9 powder, I was experiencing a somewhat frustrating lack of consistency with the OAL of the finished rounds. I had set the seating die to deliver an OAL of 1.130", but I was getting everything from 1.135" down to 1.120", and at the time I was preoccupied with how much the case might be getting shortened or lengthened by the case bulging and then DE-bulging process. At the time I assumed (without really thinking it through) that my OAL variability was somehow coming from brass of different length, and just ignored the situation. The resulting ammo behaved itself fairly well, turning in a velocity of about 1440 FPS with a velocity standard deviation of 14 FPS, which is a typical value that doesn't indicate anything is out of whack at all.

Unfortunately, Speer 125 grain Gold Dots are pushing $25/100 these days, and so I was looking around for a cheaper alternative. Came across some Montana Gold 124 grain JHP bullets for $14.00/100, and the bullet had the long, straight sides that you must have to use this bullet in 357 SIG, so I bought a 100. So I go home, use my Lyman M die (expander die) in .355" to just slightly bell the top .010" of the case, and then prime the cases. Previous trips to the range had shown that the Speer #14 manual was right on the money with it's load data for the 125 grain Gold Dot, with a workup of 1/10th grain increment loads bringing up the velocity in a nice, linear fashion to a max velocity just shy of 1450 FPS with 14.6 grains of AA#9. The manual listed it at 1437 FPS, so it was nearly dead on.

Due to the problems with base bulging at 1400+ velocities, however, I decided to be nice to my brass and go down to 13.8 grains of AA#9 with the Montana Gold bullets. So I charge powder into 50 cases in a block of 100 primed cases, and set my seating die to deliver a cartridge OAL of 1.140" which is the max for this caliber. Just as I had hoped, with this OAL the straight wall of the bullet was still sligthly protruding above the case mouth, so I could add a .003" taper crimp with a Lee factory crimp COLLET die. (Remember, this is a bottleneck case, and NOT a straight wall!!) So I get the seating die set up, and start seating bullets. Once again, I start getting variability in my cartridge OAL.

Whiskey Tango Foxtrot???

This is really starting to bug me, and I grab another case, and seat the bullet. For some strange reason, the seating pressure is really low, and the cartridge OAL is all of a sudden 1.112"!! :angryvillagers: I try 4 more cases, get 4 more light seating pressures, and an OAL of 1.112" :hmm: Then I notice that in my distracted state, I had gone down the left side of the load block instead of across it, and had loaded up 5 rounds WITH NO POWDER IN THEM!! What?? With no powder I get an OAL of .030" LESS??

About that point, it dawns on me that the only variable that has changed is the powder in the case, and I come to the conclusion that I must be compressing the powder to make up for the difference in seating pressure. Once I realize this, it THEN occurs to me that because of the pressure of the compressed powder in the case, I am getting DIFFERENT OAL's that are PROPORTIONAL to the seating pressure.

After some additional fiddling around, I come to an even more unsettling discovery. The OAL is PROPORTIONAL to the SPEED of the press handle!! A quick press stroke procuces a longer OAL that a slow and deliberate press stroke.

Unsettling discovery #2, consistent with #1. Two quick press strokes produce a shorter OAL than 1 quick press stroke.

Putting all this together, what appears to be happening is that the compressed powder is acting like a spring, and keeping the bullet higher up in the case that what you would get with an empty case, by a whopping .030"!! To make matters even more fun, this compression appears to be time dependent, so the longer it takes to seat the bullet, the more the spring effect of the powder goes down, along with the OAL.

From a purely "FEEL" standpoint, it became clear that 2/3rds to 3/4 quarters of the seating force was the pressure of compressing the powder, and once I figured this out I could "double tap" the seating on a hard seating bullet to get the cartridge OAL close to 1.140" almost every time.

About this time I recall a similar peculiar instance with reloading my 50GI cases with a measly 8.0 grains of Herco for the +P load, and getting similar OAL variations. In this case, there appeared to be PLENTY of space left in the case, but a Speer 300 grain 50 caliber Gold Dot apparebntly took up more space that I thought. A that time, I had surmounted that problem by using a slow, deliberate press stroke, without realizing that i was working with a compressed powder charge.

Obviously, this problem/phenomena will be extremely dependant on thre things:

The exact weight of the powder charge. There will be some statistivcal variations here.
The exact volume of that cartridge case. There will be some statistical variations here.
The amount of free air in the settled (but not compressed!!) charge. Obviously, a flake powder like Herco will have more entrained air than a fine grained powder like AA#9, so it can be compressed somewhat further wiout this weird spring effect taking over.

That being said, however, it would appear that if you start getting different cartridge OAL's after locking down your seating die, you may be working with a compressed powder charge, even though your manual isn't telling you that your are in this zone!!

So be safe out there, and keep your eyes open for this situation!!

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JimKirk
March 9, 2010, 10:46 PM
Thanks for the heads up. The Gold dots are scarce as hens teeth and high $$.

I have been using AA#9 with my 357 SIG too. I worked up to 13.5 grains of AA#9 under the 125 GD, I never got any such OAL variance like you did, I guess that the smaller charge did not give the spring back that the larger charge did. I have now backed down to 13.0 grs. under the Hornady 124 XTP. Forgive me as my log is out at the reloading shed, but I believe that I chrono'd that 13.0 gr. load at 1380 fps, which shoots well enough for me. I have been thinking about ordering some of those Montanna Gold 124 JHP too, do you think there could be enough difference between the bullet shapes to account for more of the bullet being in the case at the OAL than with the GD? Could that be a part of the spring back... the larger charge combined with more of the bullet deeper in the case?

Jimmy K

Seismic Sam
March 9, 2010, 11:08 PM
I think there is enough difference in the point of the Montana Gold (being slightly "pointier") than the Gold Dot for the main body length of the MG to be SLIGHTLY more than a GD, but not much. I will see if I can do some compatitive measurements and post them. I have also discovered that Winchester 357 SIG brass runs a good 30 FPS faster (average for 50 rounds) than the Speer Lazy-S brass. BTW, the Lazy-S brass apparently means "small, non-standard primer hole size" :cuss: , and the word I got third hand was that Speer's defense ammo was running a tad too hot, so they reduced the size of the primer hole to jury-rig it. I could not believe how much pressure it took to resize a Speer Lazy-S case, and after I found this primer hole information out, I could not believe that my decapping pin had hung in there and not broken while it was opening up the primer hole by a good 10 mils with simple brute force. Now I drill out the Speer brass primer hole with a .081" (#46) drill to make it equivalent to the Winchester brass.

FWIW, the Lee Crimp die for the 357 SIG is a four-fingered collet die rather than the standard taper crimp die, and it does a really nice job, and appears to be insensitive to case OAL, give or take 5 mils of case length.

JimKirk
March 9, 2010, 11:23 PM
That would be nice, I'm sure there is some difference, but I would not think too much.

I am all out of GD and all my XTP are in loaded rounds. Time to reorder. been looking at the MG bullets.

Jimmy K

Seismic Sam
March 10, 2010, 04:17 PM
Okay, here's the skinny:

http://img408.imageshack.us/img408/9863/mggdbullets.jpg

The 125 grain Gold Dot is on the left, and the 124 grain Montana Gold JHP is on the right.

The OAL for the GD is .533", and the OAL for the MG is .577". As you can see, both bullets transition from straight wall to ogive at about the smae place.

Now, if you seat a .533" GD into an .860" long 357 SIG case to get a cartridge OAL of 1.130, .268" of the Gold Dot goes into the case.

If you seat a .577" MG into an .860" long 357 SIG case to get an OAL of 1.140", .297" of the bullet goes into the case. This means the MG seats 29 mils deeper than the GD.

The volume of a .357 SIG case works out to 1.30 cubic centimeters.

The volume of a .355" diameter cylinder 29 mils high is .047 cc, which would be a 3.6% decrease in volume of the case for the Montana Gold bullet.

While I would NOT be comfortable making this swap with a max power load, the 13.8 grains of AA#9 I am using is .8 grains under the max, so it should be okay. I will chrono the loads to check the velocity, and get back to you.

Walkalong
March 10, 2010, 05:04 PM
Most folks aren't using compressed loads, so they don't see this phenomenon.

Any time you are using a compressed load of very dense powder this "spring back" can happen. The only cure is less of the same powder, lighter cases with more capacity, or a different powder.

rfwobbly
March 10, 2010, 07:10 PM
This is a problem I've also encountered when using 147gr bullets in 9mm. You've got to make sure to use a fairly dense powder on some of these loads!

After discovering powder compression I did 2 things...
• Started recording bullet lengths in my reloading notebook. With that data and some simple math you can calculate how far below the case mouth any OAL will seat any bullet.
• Made a table of how far below the case mouth different weights of all my favorite pistol powders measure. Sounds daunting, but that's only 2 or 3 powders over their usual load range at 0.2gr intervals, so it didn't take that long.

Using that data together you can spot a potential problem right off, before the first cartridge is even loaded.

;)

JimKirk
March 10, 2010, 07:22 PM
Sam that was what I suspected, that the bullet shape may have decreased the case capacity. I guess that when the case is full of powder, the extra -3.6% of room would make a big difference. You don't happen to have a 124 gr Hornady XTP that you could set next to the other two do you?

Jimmy K

redneck2
March 10, 2010, 07:56 PM
From a purely "FEEL" standpoint, it became clear that 2/3rds to 3/4 quarters of the seating force was the pressure of compressing the powder, and once I figured this out I could "double tap" the seating on a hard seating bullet to get the cartridge OAL close to 1.140" almost every time.
So, if you smash something hard twice it compresses it more than gently once....

Uhhhh....if you look in a lot of the manuals, it shows compressed loads (typically a small "c" behind the load value). In black powder, they typically use a drop tube to align and compress the grains to get less variation due to spring back.

You've just discovered something that was discovered 150 years ago.

It's an invitation to disaster to seat to OAL with different bullet profiles at max loads. As you've figured, you've got different effective seating depths due to the nose length. In bullets with a long nose, you'll get considerably more chamber pressure with the same bullet weight and charge. That's why rifle bullets should be seated (if possible) to ogive and not OAL.

Seismic Sam
March 11, 2010, 03:24 AM
Actually, the point I was trying to make is that hitting it twice QUICKLY is more or less equivalent to hitting once SLOWLY. In this particular case, the powder is acting like a semi-elastic solid, and it takes a certain amount of time at a certain pressure (determined by the setting of the seating die) to make the powder compress and stay that way. Work is normally defined as the integral of force over distance, but in this case I think it's expressed as pressure over time. 4 quick press strokes equals two slower press strokes equals one very slow press stroke.

BTW - love your sig line!!

Seismic Sam
March 15, 2010, 12:46 PM
Hokay!!! Here is the 357 SIG data for the Montana Gold 124 grain JHP bullets.

I was using Speer brass with drilled out to normal size primer holes, CCI 500 primers, 13.8 grain AA#9, with an OAL of 1.140" and a taper crimp using a LEE collet crimp die for a .003" TC. Gun is a SIG P226 Elite Stainless, stock except for a 24# recoil spring.

Shot 50 loads, with an average velocity of 1444 FPS. SD for velocity was 12, Higest velocity was 1470, lowest was 1415. In other words, all of the velocities were almost within 2 sigma deviations of the average, and 6-Sigma says anything under 3 sigma off the average is a snapshot of the normal variance of a process. Based on some other loads I shot at 14.1 grains with the Speer Gold Dot bullets to get about the same velocity, it looks like the 3.6% decrease in case capacity due to the longer MG bullet needs about .3 grains less powder to achieve the same results. The next time I reload MG, I'm going down to 13.5 grains of AA#9 just to be on the safe side. FWIW, my 1.140"OAL rounds fit easily in my SIG P226 mags, so an OAL of 1.150" MIGHT be possible....

JimKirk
March 15, 2010, 01:08 PM
I been shooting some GD with 13.0-13.5 of AA#9 getting around 1380-1400 FPS and some XTP with the same grain for a few FPS less. All shoot well of my Glock 31.

I found some hidden Win 540(HS6) that I am gonna give a try just to see how it works.

I've not pushed too much with this cartridge, I got to get comfortable with it before I do.

Jimmy K

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