Kaboom: The Consequences Of A Blown-Out Case

posted on October 2, 2018

We sometimes hear about, or actually witness, the consequences of a blown-out case. In semi-automatic pistols, the case usually ruptures on the underside where it does not have complete chamber support because of the cutout of the feed ramp. Thus, the gas pressure is generally directed mostly downward. The magazine, and any rounds still in it at the time, usually get the brunt of the blast force.

These ruptures usually result in damage to the magazine. Occasionally the magazine is blown out of the gun, the baseplate might be blown off, and so on. Some magazines are a total loss. The top round (or rounds) that might still be in the magazine could be damaged―the bullet ends up pushed deeper in the case, and the brass might be deformed. In rare cases, the top round may be ruptured. The grips might be cracked, and with polymer pistols, the frame could be cracked, split or otherwise mangled. Sometimes there’s no visible damage―just a startled shooter.

The shooter could be more than just startled, there is the risk of injury. Case ruptures blast hot gas and case fragments at high speed, as well as any gun parts that might be dislocated by the gas pressure. Skin can be torn or burned, perhaps worse. And it’s not just your hands holding the gun at risk. Gas, gunpowder particles, case fragments and gun parts can fly in a variety of directions, including straight at your face. People have been left with a bleeding face after cases rupture. Always wear glasses when shooting!

There are many causes of blown cases: too much gunpowder, an overcharge or a double charge. The bullet might be seated too deep either at the loading press or from setback during the feeding process. There is some debate whether setback can be serious enough to raise pressure high enough to cause a case to rupture if pressure would be within normal limits when the bullet is seated at its proper length. It might depend on how deep the bullet has been pushed, the strength of the brass and how much case support the barrel offers. This will not be debated in this article, except to say that setback can raise pressure and could be dangerous if pressure is high enough.

Weak, damaged or defective brass is another cause. Too little case support can result in case blowout even at normal operating pressure. Here, too, we have to make sure that our gun and components are up to the task.

As handloaders, we should work to eliminate the chance of a case rupture under any conditions. We must make sure our charge weights are within established safe limits, our overall length is appropriate, and there is enough neck tension and/or crimp on the bullet to stop setback. We have to do our part.

Then there are those crazy people who push the limits―who load beyond book values and push pressure to who knows what. Fools, they are! Unfortunately, I am one of them. In my defense, I load .38 Super and 9mm Luger (9 Major) to make Major power factor according to the requirements for IPSC/USPSA competition. Some of these loads have pressures that exceed established limits set by SAAMI for these cartridges. This can be done safely in guns and barrels designed for this purpose with the right gunpowders. Still, there are risks.

Load development for excess pressure Major power factor ammunition follows the same rules as it does when developing loads within standard pressure limits: start low and work up in increments, watching for pressure signs.

I followed this rule when working up a new load in 9 Major. Ten rounds were loaded with each increase in powder. The load used a 124-grain JHP bullet seated to 1.15-inch in Winchester cases. The gun was a 1911 single stack pistol with a 5-inch ramped barrel. The barrel provided sufficient case support for 9 Major. The case was not fully supported all the way to the extractor groove at the feed ramp, but it was close, and is on par with other barrels used for 9 Major.

During test firing, each fired case was inspected immediately after it was fired for pressure in the primer (Federal 205 small rifle) and for excessive bulging in the unsupported region of the chamber. If all the brass looked good, the next higher load was fired.

Brass Case Rupture
Figure 1: The offending case! It ruptured at the region of the feed ramp where it was not supported by the chamber.

All was going well. My last load was making Major, and the cases were all looking fine with no sign of excess bulging in the unsupported region. Until the last shot, number 10. Kaboom! The case blew out. This was quite a surprise, because the previous nine rounds showed no signs that they were at risk of failure.

The average velocity of the nine rounds before the blowout was 1358 fps. The velocity of the blowout round was 1046 fps, a loss of 312 fps. I don’t know what the pressure was, because this load was above published data, so it was clearly in the +P+ range for the 9mm.

What happened to the gun? The magazine was not blown out of the gun. The internals, however, did not fare well. The magazine spring was super-compressed, such that it was now shorter than the magazine. The metal Chip McCormick follower was broken into two pieces. I don’t know what kind of pressure is required to do this, but the results were impressive. There was no other physical damage to the gun.

What happened to the shooter? I was startled, obviously, and dropped the gun, though I was shooting from a bench so it was only a few inches. The gun had Hogue wrap-around rubber finger groove grips which are super tough. They were not damaged, but they flexed from the pressure blast, and my hand stung for several hours afterward. Some shooters have not been as lucky as I was, and received bloody injuries to their hands. I’ve seen pictures of 1911s with damaged grips after case blowouts. I suspect that my grips saved me from more serious injury.

High pressure compressed the magazine spring
Figure 2: The extremely high pressure compressed the magazine spring, making it shorter than the magazine. The metal follower was broken in half. The magazine body was undamaged and it returned to service with new internals.

Gas leaked past the grips and there were lines of black soot burned into the skin. And when I say IN the skin, I mean it. The black lines did not wash off even with heavy duty soap and scrubbing. The black soot was burned into the skin, and it took a couple days to go away. Those high pressure gases are HOT.

I was gun shy for a couple weeks after that. I remember from my Psychology classes that the most effective punishment is swift and severe. This qualified, and I had to concentrate hard to avoid flinching on my next couple of range trips.

Why did this case fail when the other fired cases looked okay? One possible mistake was using range brass. That’s right, all these cases were pickups. I sort pickups by headstamp so the volume will be the same, but I did not know the history of these brass cases. Some of them might have been used who knows how many times, shot in who knows what gun and at who knows what pressure. They could have been discards from another 9 Major shooter who was leaving them because they had already seen their fair share of high pressure loads. Or, they could have been once fired. Maybe it was simply a bad case that would be fine at normal pressure, but not at extreme pressure. I had no way to know.

In light of this event, I changed my procedure for selecting brass to use for 9 Major. Now I only use brass that I buy new, or are from factory ammo I shoot. This does not guarantee complete safety, since new brass can be defective, too, but at least it takes one variable out of the equation―an unknown history.

High pressure loads push guns and components to the limit. As handloaders, we put ourselves, and our guns, at risk with every round we load, whether at normal pressure or when pushing the envelope. In all circumstances, we should inspect our brass and discard any that are suspicious. Blowouts are dangerous even at normal working pressures. If the case looks odd, throw it out. They’re cheap. Fingers, eyes and guns are not. Be smart, be safe, so you can continue to enjoy this fun and challenging sport.


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