Unsupported Chamber, is it really?

[SA Friday]

OK, this is a spin off of a couple recent posts. I hear about how all of my glock 40 cals have unsupported chambers. This comes up all the time. It then traverses space and time and goes into the Glock 40's go ka-boom.

For the record, I have acutally seen a Glock 35 go kaboom. It was a double charged reload. It would have blown any gun. I have also seen about a half dozen other kabooms and they were all NOT Glocks. I have never seen a kaboom from factory ammo, but have read about a very very few. 1st gen Glock 40 S&W guns had excessive exposure of the cartridge wall in their chambers. 2nd, 3rd, and 4th gen chambers are NOT the same and are tighter.

What is an unsupported chamber? Most would tell you it's a chamber that allows for a portion of the cartridge wall to be exposed when the cartridge is fully chambered. By this definition, I suspect all semi-auto pistol chambers are unsupported. Yep, you can argue it all day long, but if you really check any pistol chamber, it can meet this definition. I chambered a round in a few guns and then marked the factory cartridge with a fine sharpie as deep as I could get it and measured from the back of the cartridge to the sharpie mark. Here's what I found.

Glock 9mm: .189
Glock 40 S&W: .200
Schuemann AET 40 S&W: .198
STI 40 S&W: .182
Colt 45 ACP: .224
Kimber 45 ACP: .246

Some would stipulate the above but only if the unsupported area is deep enough to expose the cartridge wall past the webbing (the bottom inside edge of the brass) of the cartridge. Well, lets check this out. I did some more measurements on once fired brass randomly selected from my armada like stockpile and discovered the following data.

R P 9mm Luger: .581 inner / .75 outer = .169 to the webbing
Win 40 S&W: .657 inner / .834 outer = .177 to the webbing
Fed 45 ACP: .723 inner / .887 outer = .164 to the webbing

So, is the cartridge wall exposed?

Glock 9mm: .189 (.169 webbing) = .020 exposed
Glock 40 S&W: .200 (.177 webbing) = .027 exposed
Schuemann AET 40 S&W: .198 (.177 webbing) = .021 exposed
STI 40 S&W: .182 (.177 webbing) = .005 exposed
Colt 45 ACP: .224 (.164 webbing) = .060 exposed
Kimber 45 ACP: .246 (.164 webbing) = .062 exposed

Yep, every one had cartridge walls are exposed past the webbing of the cartridge.

So what is the definition of an unsupported chamber? Well, I would call it NORMAL. The worst of the guns were obviously the 45's. As the chambers are scalloped instead of ramped, they expose more of the cartridge. The Schuemann and the STI are both off of custom made 2011 guns for competition with ramped feeding and the STI barrel clearly supported more of the cartridge. Both of these barrels are recessed on the side to accomidate the extractor claw and if they weren't they would not lock up and fire without having to expose more of the cartridge. It's the only way to get the cartridge deeper without having failures to extract.

The Glock 40 cal barrel did expose more of the wall than the two custom guns. Unfortunately, I don't have a plethora of 40 cals of other manufacturer to measure. If I did, the measurements would be above. I suspect they would not be that far off of the Glock measurements. I know the H&K would be more. I've had hands on, but haven't measured. They are generous in the chamber to say the least.

Here's the conclusion of all of this. Supporting the cartridge vs reliability is a trade off. You have to give on one to have the other. There is nothing wrong with the trade off. When things go bad in a semi-auto pistol they should not be blamed on the chamber unless the facts support the claim. Check your ammo first. Weak brass, fast powders, bullet set-back, double charges all make bad things happen. The conclusion that the chamber is unsupported is, well, an unsupported arguement (pun intended) and propagating false beliefs. Afterall, the worst chambers I just measured were designed in 1909 by JMB, and we all know that man was a genius.

[Graves]

How old was the glock you used? The newer models offer a bit more chamber support.

[Irving]

It's about time this site had a real technical discussion thread. Well done. :)

I made my post based on a similar post I read else where, where the person did basically the same thing as you did, but with pictures. I definitely remember that the HK had noticeably less chamber support than the GLOCK.

One thing that I left out of this whole chamber support discussion, is that it is better to have a round explode out the bottom of the chamber, than to have it explode your barrel, or crack your slide. A lot of those GLOCK Kaboom stories that you hear so much about (but rarely see) also say that as a result of the case wall exploding out the bottom, the magazine just blows out the bottom, and the rest of the gun (and your hand) is left unscathed.

Thanks for the correction on the generations that had this issue. I also heard some hearsay that the 1st gen GLOCK 40s were built on 9mm frames, rushed to market, blah blah blah. That tends to always be brought up, but is never proven one way or the other.

I am under the impression that the chamber support of the .45 cal is less important as it is loaded at significantly less pressure than a standard .40 cal round. This was also brought up in the other thread I had read.

One last thing, I'm not sure if I would let this information affect my purchase of a GLOCK in 40 or not. If it DID effect my decision to purchase, I would openly admit that it is just a personal thing, and others shouldn't follow suit based on my own quirkiness.

I'm glad you brought this up, as it leads to another common gun myth that I hear about, but don't quite believe. Rounds cooking off. I've heard the term before, and have heard one story in person during a shooting comp. The guy said that him and another guy were shooting AR-15s and the other guy stopped shooting, set his rifle on the stand, then it "went off." He said that the chamber was so hot that it cooked a round off. I have a hard time believing this, as it seems like while it would take time for the heat to transfer from the chamber to the round, during that same time, the chamber would also cool. It just seems far fetched to me and want to know what other people think about it.

[SA Friday]

I took a pic, but I would have to shrink it so small to post, it just wasn't worth the effort.

The Glock 9mm and 40 S&W I used in the measurements were both 3rd gen Glocks.

45 ACP chamber pressure is about 20,000 psi max
40 S&W chamber pressure is about 35,000 psi max
9mm Luger chamber pressure is about 35,000 psi max

So, yes, 45 ACP isn't pushing the brass as hard as the other two but you are still exposing annealed brass pretty fricken hard at 20,000 psi.

I have seen about half a dozen mags get blasted out of the gun from brass giving way. All were in 9mm and 40 S&W. I haven't seen a 45 blow a mag out yet, but I know the day is coming. The guns were Glocks and STI's, and it was a combo of brass reloaded past it's life cycle and fast powders. The reason it was Glocks and STI's is because they are used a lot in competition shooting, and competition shooting offers the ideal grounds for this to occur. There's lots of reusing brass and reloading with fast powders and heavy bullets. The 9mm brass was speculated to have been shot out of an MP-5 before used in one of the Glocks. MP-5's are notorious for destroying brass.

Cook offs. Yep. Seen one of those too. It was a belt fed machine gun during qualification. It happens. Pretty tough to do with an AR, but not impossible. It would be expensive (and very hot outside) to get an AR to that temp, but very plausible.

[eerw]

I really like when you get too much time on your hands :D

so more questions concerning failures in unsupported chambers.
and the design of most semi-auto handguns is for an unsupported chamber for the proper feed angle from the magazine to the barrel.

when a piece of brass is unsupported..at what pressure does one see it fail at.
what are the variances of brass thicknesses per manufacturer and per industry standard.
what is the brass consistency in construction of said brass.
what effect if any does the curvature of the brass play in strength of the material.
what effect does variances bullet depth, crimp, burn rate of powder, pressure curves, diameter of barrel have.

so at its very basic level..a given barrel, with a particular powder, bullet, diameter barrel, crimp, brass will go kaboom if you stack all the tolerances in the wrong direction.

[SA Friday]

I really like when you get too much time on your hands :D
I'll write the articles if you take the pics.[Coffee]

[eerw]

I'll write the articles if you take the pics.[Coffee]

works for me...
do i get to play with your Bennie gun?? :D

[SA Friday]

I really like when you get too much time on your hands :D

so more questions concerning failures in unsupported chambers.
and the design of most semi-auto handguns is for an unsupported chamber for the proper feed angle from the magazine to the barrel.

when a piece of brass is unsupported..at what pressure does one see it fail at.
what are the variances of brass thicknesses per manufacturer and per industry standard.
what is the brass consistency in construction of said brass.
what effect if any does the curvature of the brass play in strength of the material.
what effect does variances bullet depth, crimp, burn rate of powder, pressure curves, diameter of barrel have.

so at its very basic level..a given barrel, with a particular powder, bullet, diameter barrel, crimp, brass will go kaboom if you stack all the tolerances in the wrong direction.
ooooooh, I didn't think of that one. That's good.

One factor that I didn't address (it's a lot to get your head around) was how material spreads force. For example, When you bridge shore a vehicle in an aircraft for transport. The supported weight (force) is distributed to the contact surface of the pillar it's rested on. So, you can spread out the actual area of contact to the fragile cargo floor of heavy objects. The distribution occurs at a 45 degree angle. You also see this distribution of force when a bullet traverses glass in a 45 degree bevel in the direction of travel.

So, when the chamber pressure is distributed to the cartridge wall, how much can be exposed unsupported and not acutally bridge to either the chamber wall or the brass webbing? This also would be affected by the thickness of the chamber wall. I didn't even think about how the curvature of the cartridge would affect this.

I need a job....[Tooth]

[TFOGGER]

Not to mention that most reloaders don't anneal their brass, so the cases work harden and split, rather than deform and contain the pressure....it's pretty safe to say that factory loads are responsible for a VERY (vanishingly?) small percentage of case failures. Add to this the tendency of some reloaders to push the envelope in the name of performance...BOOM

[SA Friday]

Not to mention that most reloaders don't anneal their brass, so the cases work harden and split, rather than deform and contain the pressure....it's pretty safe to say that factory loads are responsible for a VERY (vanishingly?) small percentage of case failures. Add to this the tendency of some reloaders to push the envelope in the name of performance...BOOM
True. Factories anneal the brass before it's loaded. Not all are the same. For example, Starline only anneals their brass 3 times. It's pretty soft brass. Even working brass in a fairly liberal 40 cal chamber, you should be able to get at least a half dozen reloads out of (american made) brass. Most get even more. 9mm brass at average target load pressures can quite literally be reused until it's lost or splits.

[hobowh]

great info thanks

[Colorado Osprey]

I've only ever witnessed 2 KB's.

Neither were Glocks... but both were 40S&W

I think it is an issure primarily with the cartridge's higher pressure. If the Glock chamber was truely the issue, why don't the 10mm's explode?

The 2 guns that chunked were a Ruger P91 and a Beretta (Italian) 96.

Both guns were sent to the manufacturer and replaced by the factory.

The ammo was commercial re-man training ammo by different comapnies about 5 years apart.

[Hannu]

As an engineer, I just need to say something techical :)

Supported / unsupported chamber is an interesting question. I would say, we can not compare different calibers because they have pretty different case design. For example, 9mmx19 has lot's of taper but .40 does not.

I used to have a Glock 35 .40 S&W years ago. Chamber of that pistol had pretty much taper. Because of that, brass life was very limited, as .40 S&W case does not have much. We made a special tool similar to this : http://www.midwayusa.com/viewproduct/?productnumber=358543 and case life was extended.
However, with match loads (heavy bullet + fast powder) there were some case / firearm breakages.

I was told, Glock had much tapered chambers because of feeding. .40 S&W has very little taper, which makes it more difficult to feed compared to, for example, 9mmx19.

After we got first STI .40 S&W pistols, brass life was extended pretty much. With case head sizing tool, I used my training brass some 15 - 20 times with my first STI Edge. After that, those cases could still be used in Glock couple times.

I would say, (once used) case head diameter just front of the extractor groove should be .427" or less if you are planning to use your cases for a long time. .425" or less would be even better, but pistol needs to be tuned very carefully so it will work.

We need to keep in mind, that many match loads generate clearly more pressure than CIP or SAAMI maximum pressure.

I would say, with factory loads there is no problem if chamber is little loose. But, if you want to reload your brass many times and want to use high pressure loads, loose chamber rear end is a problem.

Just my 2 cents :)

[Irving]

I took this picture sometime last week to help illustrate what this thread is talking about, in case anyone was confused. Sorry it took so long for me to put it up.

This is a .40 S&W seated in the chamber of the barrel of my Smith & Wesson M&P 40 compact.


http://i450.photobucket.com/albums/qq226/1stuart1/casesupport.jpg