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Old 12-25-2005, 07:11   #266
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Join Date: Dec 2005
Location: London
Posts: 6,079
Cause of KBs

This is clearly a very popular thread and I have amazed myself by reading it all. Rather than go back and find all the references I will just refer to the ideas.

One of the most striking things is the aggressive interaction initiated by the man (avatar of Tasmanian wolf) who has had twenty Glocks but thinks that they are more likely to blow up because they have sloppy chambers. His extremely aggressive initial approach triggered everyone else into a fight mode rather than a rational problem solving mode. He has done the thread a great disservice. His principal belief that loose chambers cause blowouts has to be nonsense. All cartridge cases expand to seal the chamber, more or less, regardless of how sloppy the chamber is and then spring back most of the way. A sloppy chamber will produce a bigger diameter “empty” than a tight chamber. The expansion happens in the early phase of the ignition because the brass is very weak in comparison to the chamber. Any “slap” of brass on chamber at this stage must be negligible compared to the later rise in pressure. If anything a sloppy chamber will reduce the total pressure because it provides a bigger space for the “burn”.

All barrels must be designed to some compromise with respect to their resistance to exceptional events and all will blow up given the wrong, or right, circumstances. I doubt that the G21 is more likely to blow up than the G20 because, as asserted by the same man, their exterior diameters are the same but the hole in the 45 is bigger than the hole in the 10mm. The standard pressures are nearly twice as high in the 10mm and so the 45 has less likelihood of a charge big enough in the right circumstances. When we come to the .40S&W in comparison to the 9mm he might have a valid point. The working pressures are much the same but the chamber wall is thinner in the 40. I don’t believe it but I could be convinced by appropriate evidence.

I have long believed that all manufacturers made a serious mistake by treating the 40 as a bigger bore 9mm just because it could be fitted into the same frame. It shoots bigger bullets at the same velocities and so has more recoil. It needs a correspondingly heavier slide and barrel to slow the slide recoil velocity. It is significant that the accuracy of the .40S&W from a revolver is inherently high but from an auto it is poor compared to other autos. I believe excessive initial slide velocity is the cause and initial slide velocity is hardly affected by spring strength. I think Glock should go the same way as with the GAP45 and increase the slide weight but I don’t think that this has any connection with KB incidents.

Why do we care whether the chamber is tight or loose? For best accuracy we want a tight chamber and a tight slide but a relatively small amount of dirt or grit can cause a failure to feed or failure to go completely into battery. This is not just because there is more friction in the chamber. The timing of the rise of the cartridge depends on both the strength of the magazine spring and the speed of the forward motion of the slide. If the slide is slowed by dirt the magazine spring will give the cartridge too much vertical velocity relative to its horizontal velocity. The result will be that it is too high by the time it reaches the chamber mouth and can jam against the upper edge of the chamber. A slightly looser chamber is not so sensitive to this timing problem and will also put up with a more angled presentation of the cartridge. If you want a reliable defensive handgun that keeps going bang when you pull the trigger until the magazine is empty you need a slightly sloppy slide that does not get slowed down by dirt and a slightly sloppy chamber. If you want maximum accuracy then you want both to be tight. There is no “best” solution – it’s a compromise. In this case, versatile as they are, the Glock pistols are strictly designed for combat use which has to include difficult conditions. They don’t need to shoot into 1 or 2” at 25 yds because 4” is good enough for the purpose. If you want to reload, get better accuracy or shoot lead bullets then you need an after market barrel made for that purpose. If you want service reliability and acceptable accuracy for the purpose with a little more velocity for a given pressure then you want a Glock polygonal barrel and the bulged cases don’t matter.

There is no indication that the so called unsupported barrel causes any other problems (if you don’t reload) because the case wall at that point is thick enough not to burst in a normal firing event. If you block the barrel with a squib load and then fire another normal cartridge you just might be lucky enough for this section to blow out while the barrel is still locked to the slide and this might just save you from a far worse barrel burst. Was it designed as a safety valve? I doubt it! It was designed to give a shallower ramp into the chamber which in turn improves reliability. Could the barrel have been located a little further forward at the cost of miniscule extra length of the pistol? Probably! Would it have been better if it had? Possibly. Was it therefore a design fix in late development? Could be. Does it matter in the least for a combat pistol? Not a jot!

The man who did the entirely sensible experiment of successively shortening a 10mm case to find out whether his .40S&W Glock would fire out of battery had it right but I don’t think that is part of the KB problem. If the cartridge is far enough out of the chamber to be dangerous the striker will be blocked in all but very rare cases. In this category of rare events the cartridge would be ignited before it is properly supported and the result would be a burst cartridge case not a burst barrel because the case would burst long before the pressure reached a level which would endanger the barrel. There has been no evidence put forward of any such mechanical failure of the striker safety block being the source of a case blow out.

The head of the Portland Police Bureau should have been sacked over the G21 KBs. Perhaps he has been by now. To have two failures within two days when the last was five or so years previously has to be an ammunition problem. It was all the same batch of ammunition and not their normal ammunition but the guns had been working for a long time before that training day with no problem. Coincidences do happen but you shouldn’t rely on them. The fact that one failure was at the end of the day and the other was only 20rds after cleaning also says that it was not a build up of dirt in the conventional sense. Those who start with preconceptions are likely to make costly mistakes. To carry on shooting after the first KB without determining the cause seems close to criminal negligence. To try to put the blame onto Glock seems to be either a case of passing the buck or culpable stupidity.

I like the original theme of the thread which distinguishes between a barrel splitting incident and a much less dangerous burst case incident but I will shortly explain why I think that many of these incidents have the same cause.

The barrel and slide start moving back together as soon as the bullet starts moving forwards. Under normal circumstances the barrel does not cam down out of breech lockup until the bullet has cleared the barrel and the gas pressure is virtually zero. This obviously happens very rapidly and the empty cases of my G20 10mm show a smear of the firing pin indentation on the primer so the barrel is camming down before the striker has rebounded.

How can a barrel blow up? The simple but unhelpful answer is because the pressure is higher than it was designed for or it has been weakened in some way irrelevant to this discussion. So the proper question is how does the pressure get so high that a barrel can blow up? This takes many times the working pressure! There are two ways. One is that the cartridge has been over charged for the propellant used. The other is that it is burning in too confined a space. I think that the first is rare in auto pistols because they use small cartridges designed for smokeless powders and so do not have much room for a double charge, which is the most likely reloading error. This is much more likely with what are actually old fashioned revolver cartridges designed originally for black powder and which therefore have more spare volume.

The simplest form of “too small a space” is caused by a load with no propellant, or very little, which leaves a bullet part way up the barrel. The next bullet then comes to an abrupt stop but the powder keeps burning even faster than normal because it is hotter, and the result is an explosion. Bear with me for re-stating the obvious – for those new to such topics it is necessary and I am getting to the denoument as fast as I can. Likewise if you load a very heavy bullet with a powder and charge designed and tested for a light bullet, the space available expands too slowly, the pressure builds too rapidly and damage is likely to follow. Even if you crimp the bullet too tightly in the case the extra time it takes to start moving can be enough to cause a dangerous build up of pressure as the powder has extra time to “burn” before the space available to it has opened up enough to both cool its temperature, and therefore rate of reaction, or to reduce its pressure by containing it in a bigger volume.

How else can we achieve this undesirable effect? If a previous bullet or bullets leaves a substantial deposit part way down the barrel, then the next bullet will slow down enough for pressure to become high enough to cause damage. The actual damage will depend on how much it is slowed and how far down the barrel the obstruction is located but in any case the bullet can end up leaving the barrel and taking the obstruction with it. The PPB should have looked for it! Remember that the barrel and slide have started to recoil together and that the engineer who designed and tested it expects the bullet to have left the barrel before the breech unlocks. But is this case the bullet has been slowed down even though the pressures are much higher than he expected and his calculations and testing are worthless.

If the partial obstruction is a short way down the barrel the recoil will have been reduced at an early stage and the slide velocity will be low. The breech will still be locked as the pressure builds to a dangerous level and the barrel will bulge or burst.

If the obstruction is further down the barrel the pressure will be lower because there is more space and so a burst barrel is less likely but a more important effect will have taken over. The bullet has moved further and faster before meeting the obstruction and so will have produced more recoil. This will have produced more slide velocity which will have unlocked the breech. The high gas pressure will be forcing the cartridge case walls against the side of the chamber very hard so the probable result is that the head of the case blows off taking the extractor with it but leaving the rest of the cartridge expanded tight in place against the chamber. This is because the chamber has actually been swelled outwards by the excessive pressure but has than sprung back to grip the walls of the case when the pressure fell. This fits the picture of the barrel shown earlier in the thread.

This hypothesis fits the facts and provides a single explanation for many of both types of explosive event. The problem is an ammunition problem and is nothing to do with the weakness of the barrel or the unsupported section. As to what manufacturing fault of the bullet can leave such an obstruction, I cannot guess. Perhaps it needs a rare sequence of failures.

I believe that case blowouts are much more likely to be caused in this way than by a failure to go into battery combined with a failure of the striker safety block. If I were a proper engineer I suspect that I could do calculations based on the pressure resistance of the exposed case and the limitations on bullet velocity that would be created to show that this was so.

The moral of the story is that we should use good bullets. The difficulty with this is that, for all we know, some apparently good and expensive bullets might be at risk of this occasional failure. What we need is a data base of KB events, including case blow outs, so that we can see if there is any consistent pattern to implicate particular bullets or firearms.

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