9mm Luger Case Volume

[Mark K]

Living in Colorado Springs now, wife and I did some shooting (mostly with reloads) a couple days ago up in the mountains. :-)

I'm now processing the brass, as much of it as I could find in the pine needles. Out of about 100 9mm Luger shell casings that I fired, I noticed one that wasn't like the others (see attached).

It has a "shelf" all the way around the inside wall, about 2/3 of the way down toward the case head. All the other cases I ever remember seeing have been straight-walled all the way down.

Its headstamp, besides 9mm Luger, shows *IMT*. So apparently Turkish Anatolia Cartridge Industry Co. LTD.

Seems like that interior narrowing would significantly reduce the internal volume of the case. Would that cause a pressure issue?

Or does the manufacturer compensate for that...? In other words, how "standard" are the internal-volume standards for 9mm Luger worldwide?

This was obviously one of my range-pickup reloads. No overpressure signs on the case -- the primer's already gone.

Since this is the only case like this out of ~100, I'd be glad to trash it. I just need to know if I have to pay much closer attention to headstamps in the future...

 

[ohen cepel]

Everyone I know pitches them. Worries me that one will slip into my reloads by mistake.

I can see them ramping up pressure a lot if loaded like a standard round.

 

[Biggfoot44]

Yes , there are wide variations in 9x19 brass from mfg to mfg .

 

[Mark K]

Yes , there are wide variations in 9x19 brass from mfg to mfg .

Everyone I know pitches them. Worries me that one will slip into my reloads by mistake.

I can see them ramping up pressure a lot if loaded like a standard round.

Yes, but enough to cause a significant safety issue if loaded within specs?

Everyone I know pitches them. Worries me that one will slip into my reloads by mistake.

OK, I'll pitch this one. Wonder how common they are, particularly these days...

*sigh* At my age, it's easy enough to detect case defects. But it's getting tough to peer at each headstamp... :-/

 

[Biggfoot44]

" Specs " only apply to the exact combination of components listed . If Acme Powder company published 9mm data using XYZ brass , and you used PDQ brand brass that had 10% less capacity , that could be a serious issue .

This is particularly a thing with 9x19 . It is mfg all around the world both Civillian and Military , and it is a high pressure ctg , and with inherently small case capacity ( compared to say .38 spl) , small differences in absolute measure can be meaningful as a % .

 

[Mark K]

" Specs " only apply to the exact combination of components listed .

Very true. And many sources specify the cases and even primers used. Plus, of course, chamber pressure and muzzle velocity will vary with barrel length.

But load data have to be valid across a wide spectrum of variables. That's why I believe load data are conservative enough to allow for these variations.

You're absolutely right regarding 9x19. Military loads are hot, and military submachine gun loads are even hotter...

 

[Biggfoot44]

Bbl * length* doesn't effect chamber pressure .

Chamber size , throat , and leade Do .

That's why most test barells have exact minimum SAAMI ( or CIP , etc ) chambers . That way pressures in standard production gun will always be equal or less than test pressures .


Trivia - The chamber specs for Wetherby ctgs include a long throat by design . If a 'smith uses a shorter throat in custom guns , it could cause pressure issues with factory ammo , or jumping to upper end data without working up from well below .

 

[Mark K]

Bbl * length* doesn't effect chamber pressure .

Well, barrel length should affect the pressure that can build up (not just in the chamber) before the bullet exits the muzzle. Or does a firearm reach max chamber pressure before the bullet exits...?

With a short barrel (my SigSauer P290 RS only has a 2.9" barrel), there's a significant fireball at the muzzle. That indicates that powder is still burning, and I'd think that pressure would still be rising...

Why do SAAMI specs specify the barrel length(s) used for various calibers?

 

[John from MD]

Well, barrel length should affect the pressure that can build up (not just in the chamber) before the bullet exits the muzzle. Or does a firearm reach max chamber pressure before the bullet exits...?

With a short barrel (my SigSauer P290 RS only has a 2.9" barrel), there's a significant fireball at the muzzle. That indicates that powder is still burning, and I'd think that pressure would still be rising...

Why do SAAMI specs specify the barrel length(s) used for various calibers?

I have several books on internal and external Ballistics but I'm afraid I am not good an shrinking thousands of words and equations into a few sentences.

I'll let Bigfoot have a shot at that job.

 

[Biggfoot44]

Pressure peaks in the chamber ( or just barely starting to move , after that the pressure is falling .

That said , a shorter bbl will have a higher muzzle pressure , with louder blast , more fireball , etc . A longer bbl will have greater total area under the curve , but lower pressure at the muzzle when the bullet exits .

This is in the context of " normal " pistol bbl length . If you read various chrono tests with carbines , SBRs , and SMGs , typical 9mm ammo will reach max velocity at +\- 12 inches , and sometimes will actually slow down in a 16in bbl .

Added- SAAMI , CIP , etc has a specified standard test bbl length , for consumers to have a semi- realistic idea of expected velocities for typical pistols .

 

[Mark K]

Pressure peaks in the chamber ( or just barely starting to move , after that the pressure is falling.

Sooo... we're digressing a bit, but this is interesting... Chamber pressure peaks just as (or barely after) the bullet starts to move. Makes sense.

Is that because most of the burn is complete; or because the bullet's movement starts opening up the available volume?

Powder is still burning, and may continue to burn even after the bullet has left the muzzle. But the pressure is still falling, because most of the powder has ignited and the total chamber + bore volume increases as the bullet moves down the bore.

And, of course, as long as there's any significant pressure behind the bullet in the bore -- even though the pressure is falling -- the bullet will continue to accelerate until it's cleared the bore. And that, of course, is why barrel length is so important.

Yes?

 

[Doco Overboard]

Sooo... we're digressing a bit, but this is interesting... Chamber pressure peaks just as (or barely after) the bullet starts to move. Makes sense.

Is that because most of the burn is complete; or because the bullet's movement starts opening up the available volume?

Powder is still burning, and may continue to burn even after the bullet has left the muzzle. But the pressure is still falling, because most of the powder has ignited and the total chamber + bore volume increases as the bullet moves down the bore.

And, of course, as long as there's any significant pressure behind the bullet in the bore -- even though the pressure is falling -- the bullet will continue to accelerate until it's cleared the bore. And that, of course, is why barrel length is so important.

Yes?

Sounds like you You pretty much have it. One thing to keep in mind is that as the bullet travels up the bore eventually it wins the race over the burning powders ability to make heat which equals work as the size of the chamber has now essentially became lengthened.
What affects pressure is burning rate of the powder, fit of the bullet into chamber seat, chamber specifications (neck) hardness of the bullet, length, weight and bearing surface of the bullet, friction of the bore, the bullets ability to maintain a gas seal
The bullet shape upon forcement, hardness of the bullet and of course length of the barrel are also important factors to consider.
The longer the barrel there is more time for pressure to act against the base of the bullet to a point where friction begins to reverse the process which you already mentioned. Thats why there is an optimal barrel length for cartridges such as the lowly .22.
A test barrel at the Springfield arsenal produced 42,000 lbs of pressure measured with a strain gauge near the bullet seat and another located near the muzzle indicated a residual of 8,000 lbs before the bullet quit the muzzle entirely.
Somewhere in some old dusty treatise on ballistics is a breakdown of the different components of the bullet, brass cartridge case composition, transfer of BTU's through each to barrel reinforce, bolt, receiver etc.

In regard to the picture you posted of reduced powder capacity of the case you provided is what appears to me to the shape of a brass cup during cartridge manufacture process as the second step of drawing out a case from a tube.

 

[Mark K]

One thing to keep in mind is that as the bullet travels up the bore eventually it wins the race over the burning powders ability to make heat

That's something I hadn't thought of. The bullet keeps accelerating, because the pressure behind it is so much greater than in front.

But that rapidly-increasing volume behind the bullet would also tend to cool that volume really fast, at least relatively. That's qualitatively... I wonder how much quantitatively...?

And how much does that contribute to the instant cooling (and therefore necessary shrinking) of the brass case to allow the chamber to open...?

 

[erwos]

Maxxtech 9mm brass is infamous for this shelf. It's really not a bad design on its own merits - less powder for the same pressure, better case head support, and you know you're never getting bullet setback. Problem is when you've got it mixed in with normal brass.

 

[lazarus]

That's something I hadn't thought of. The bullet keeps accelerating, because the pressure behind it is so much greater than in front.

But that rapidly-increasing volume behind the bullet would also tend to cool that volume really fast, at least relatively. That's qualitatively... I wonder how much quantitatively...?

And how much does that contribute to the instant cooling (and therefore necessary shrinking) of the brass case to allow the chamber to open...?

You are looking primarily at pressure vs friction. If the pressure on the rear of the bullet is higher than the friction of the bullet traveling down the barrel, it will accelerate.

Some powders are hotter than other powders for the same pressure produced. Pressure rise generates heat (ideal gas law) but heat can also generate pressure (again, ideal gas law). But some of the pressure is generated because you are turning a solid in to a gas and some of the pressure is generated because of the heat put in to the gas you are generating.

Hopefully that makes sense.

Peak pressure is generally just after the bullet has engaged the rifling and begun to travel down it. Which is why a throat that is too short can cause a pressure spike, because the bullet barely moves before it hits the lands and grooves and suddenly has to force its way down the barrel. This allows the pressure to build and the powder to combust at a faster rate accelerating that pressure build.


If the bullet has further to jump, that increases total combustion volume more, which reduces the combustion rate so the bullet eases in to the barrel more.

All relatively speaking of course. Both are happening in scores of microseconds as the primer begins to ignite the powder.

As for the exact peak, it’ll depend on case crimp, throat length, Bullet seating depth, powder type and powder composition (shape). A really slow magnum rifle powder with a long throat and long seated bullet might reach peak pressure 2-3 inches down the barrel. A super fast pistol powder on a magnum cartridge revolver with a heavy crimp might reach peak pressure as the bullet is leaving the case and before it’s hit the forcing cone.

As for the specific case example from the OP, could measure the water volume of the case VS a few other examples. It isn’t going to be strictly a linear function as reduced volume will also speed combustion. But if you lose, say, 5% case volume with everything else being held the same you are probably increasing pressure 10-15%. You could plug it in to quick load or GRT to get a much better guesstimate.

Looking at it, if you were already running it to +p+ pressures that would be extremely dangerous. If it was loaded to a mild plinking charge, nah, I doubt that would have even gotten you to +p pressures.

It varies, but IIRC for 9mm most manufactures is pretty high. For NATO it is 45,700PSI ( it is set at 25% over rated pressure). And remember, a proof load should allow the firearm to handle it with no damage.

It might cause accelerated wear if you run hundreds or thousands of rounds at those pressures and might eventually fail. But a round or two at or around that should be okay.

NATO pressure is 36,700. +p is 38,500. Standard is 35,000.

+p+ has no actual rating. Just means it exceeds +p ceiling.

Most guns that are rated for +p are LIKELY tested to a 25% proof rating. So likely around 48,000psi.

Not sure if that makes you feel any better. Part of the reason why if you are loading plinking range ammo, load it on the lighter side if it runs. Then you can load it sloppy. Not careless, but so you don’t care as much about tolerances. Or you can go all over the top about ensuring you are using match cases, 32 step process to recondition and anneal the brass on each reload. Set neck tensions, trickle the powder, etc ad nauseum.

I don’t load a lot of stuff super hot. But when I am there loading some 147gr right at the limit of what my reloading manual and chronograph is saying 9mm (or 45 or whatever) is rated at, I might darned sure to use all the same brass, make sure those flash holes and pockets are clean, double check COAL somewhat frequently, make sure all rounds pass a plunk test, double check powder loads every once in awhile while loading, etc.

If I am loading up some mild 115gr that’s just going to piddle brass out the side of the gun I just make sure each case has powder in it before I seat the bullet and move on to the next one. I am not really super negligent about it, but so what, I checked the first couple of cases. If my powder measure is throwing an extra tenth or two if a grain by the end, meh. I am like a grain under max charge. If the COAL is varying .020” and it’s mixed brass varying in volume by 2-3%. Meh. Again, I am way under max pressure it’s no chance of getting me over pressure.

I am still going to put most of the magazine in a coffee can at 12yds shooting reasonably fast and can put most of the magazine in to a torso sized target at 50yds shooting carefully. Oh sure, being all careful about how I load the ammo does increase my accuracy some, but I am more the limiting factor than my RELOADS.

Take apart a box of factory range ammo and make sure to tie your jaw in place, because it’s going to be BAD. I’ve only pulled apart a few rounds out of curiosity, but what I’ve seen isn’t far off what I’ve read. You can have BIG variations from one round to the next (several tenths if a grain, COALs varying a lot, several grains difference in bullet weight). Just run it over a chrono if you don’t want to tear it apart. Most commercial loaded range stuff I get SDs around 60fps for 9mm and I’ve seen extreme spreads of more than 100fps. My “shoddily” loaded plinking stuff is usually around 25fps for an SD and IIRC I see less than 40fps ES. and my carefully loaded stuff is usually around 10fps SD with maybe a 25fps ES.

Defensive ammo seems much more consistent for sure.

 

[Biggfoot44]

The other important factor is the friction of the bullet passing thru the bbl , it requires ( a certain amount of force) to overcome the friction of pushing the bullet . In a long enough bbl , eventually the pressure of the gasses drops to lower than the friction , and the bullet will eventually slow down and stop . Witness my example above with some 9mm ammo having lower velocity from 16 in than 12 in bbl .


But without getting deep in the weeds . and over my head in the physics and chemical engineering of it all , you have the gist of it .

 

[Mark K]

Looking at it, if you were already running it to +p+ pressures that would be extremely dangerous. If it was loaded to a mild plinking charge, nah, I doubt that would have even gotten you to +p pressures.

It varies, but IIRC for 9mm most manufactures is pretty high. For NATO it is 45,700PSI ( it is set at 25% over rated pressure). And remember, a proof load should allow the firearm to handle it with no damage.

It might cause accelerated wear if you run hundreds or thousands of rounds at those pressures and might eventually fail. But a round or two at or around that should be okay.

Yeah, that's what I figured/hoped. I'll still trash these when I find them, but won't particularly sweat it. My 9x19 loads are pretty middlin' (115gn FMJ with 4.5gn of HP-38), and they cycle all my 9x19 handguns just fine.

Take apart a box of factory range ammo and make sure to tie your jaw in place, because it’s going to be BAD. I’ve only pulled apart a few rounds out of curiosity, but what I’ve seen isn’t far off what I’ve read. You can have BIG variations from one round to the next (several tenths if a grain, COALs varying a lot, several grains difference in bullet weight).

I've been wondering about that too. Wow.

 

[KRC]

Boyles Law:
P1V1 = P2V2
As volume increases (V1 to V2 as the bullet moves down the bore) P2 must decrease (P1>P2).

This is overlain with additional generation of pressure by continued burning of the powder as the bullet moves down the bore.

Also Ideal Gas Law: PV = nRT
I once had a "barrel burner" chambered in 6.5x300WWH (an old 1000 yard chambering), and as the barrel went south it opened up not at the throat, but in the middle of the barrel due to the slow burn of 94gr of H870 which seemed to achieve the maximum corroding temperature in the middle of the barrel.

 

[Pinecone]

Boyles Law is over shadowed by the burning powder. It deals with a constant amount of gas.

A lot depends on the burn rate of the powder.

Garand loads are one place this is very important. A faster power burns earlier and creates a higher peak pressure and then the system acts like Boyles Law. Realizing that the lowering pressure due to increased volume may still get enough to accelerate the bullet. Slower powders are burning longer so lower peak pressure, but the pressure is higher longer.

In a Garand, there is safe pressure limit when the pressure hits the gas port. Slower powders produce that higher port pressure for the same muzzle velocity.

 

[John from MD]

Boyles Law is over shadowed by the burning powder. It deals with a constant amount of gas.

A lot depends on the burn rate of the powder.

Garand loads are one place this is very important. A faster power burns earlier and creates a higher peak pressure and then the system acts like Boyles Law. Realizing that the lowering pressure due to increased volume may still get enough to accelerate the bullet. Slower powders are burning longer so lower peak pressure, but the pressure is higher longer.

In a Garand, there is safe pressure limit when the pressure hits the gas port. Slower powders produce that higher port pressure for the same muzzle velocity.

You forgot to mention what the slower powder does to a Garand op rod.