Or more specifically, why don't they burst? You have huge pressures behind them, they're thin and not really solidly fixed in place, and yet they are expected to stay intact - how?
Me and colli were discussing this on TS yesterday, and there's no way inertia of the firing pin could be providing meaningful support - from squinting at a pressure Vs time graph of a 30.06, and assuming the firing pin can be modelled as a piston of radius ~1mm acted on by the chamber pressure, the applied impulse is on the order of kgms-1. So that firing in should be moving backwards at a considerable velocity, given the light weight, and over the ~1millisecond of the firing process it should move several mm back - yet primers retain the impression of the firing pin after firing. The peak force applied was on the order of kN, so I'd be very surprised if the firing pin spring was providing enough of a force to resist this
First off, notice the "might" in the title. It is not yet known what exactly happened. What I'll be talking about is something I heard from someone close to the people involved. It might turn out to be not true, or it might be true. To be sure we have to wait for the official report of the investigation.
Second, it might seem I am attacking the victims of the accident, this is not the case. But if they made mistakes, I will point them out.
So most of you have probably already heard of the accident with the M18 Hellcat. What I have heard from people close, is that the round went off when they opened the breech after a misfire, or slightly after they opened the breech. So, a misfire huh. Nasty stuff when it involves explosives.
So, what happened?
Well, misfires happen. There's nothing strange about that. I assume a lot of you have experienced misfires with small arms, and you know the procedure of dealing with them. But with misfires like these are handled (completely) differently. I asked around a bit, and apparently the gunner waited a few minutes after the misfire before he opened the breech. This is good, but not good enough. Not by a long shot. If I remember correctly, when your small arms firearm misfires you keep the barrel pointing down range for at least 30 seconds. After 30 seconds you can safely assume the round will not go off by itself. It's different when a proper amount of explosives is involved. You do not wait 30 seconds.
You wait at least 30 minutes. But between a misfire and waiting is another step. But I don't know if that step is possible on a Hellcat. More modern tank guns have two firing systems. The normal one, and an emergency one. If there was a misfire you were supposed to try the emergency firing system next, and if that didn't work: Time to wait.
After waiting 30 minutes there are two things you can do. The first is to open the breech and check everything. Carry the round to a safe place, and blow it up. This is usually what you can do with normal, proper rounds. But in this case, with more shady ammunition I would go for option two: Call Ordnance. There are multiple things that could be wrong with the round, and I'm go out on a limb here and claim that the gunner did not have Ordnance training. In the military, if something goes wrong, Ordnance immediately becomes the supervisor of everything that happens. There might be Generals running around, but that mere Sarge (or whatever rank they have in the US) is in charge.
This is what Ordnance would most likely do:
- Establish what round is actually in the gun. Is it an original WW2 round, or is it aftermarket? What primer did they use? What powder? Is it an AP shell, or HE? Does the shell have a fuse? If yes, what type of fuse?
- Try to establish what happened with the round before it went into the gun. How was it stored? Did you put it in your shed, or in a bunker with AC?
This is all to determine one thing: Is the round stable? In other words: Can I move the round?
If the round is determined to be stable, Ordnance can do two things.
1. Open the breech from a safe distance, and making sure the round will be caught before it hits something. Considering an historic piece of equipment is involved, this can result in the best possible ending. Which is a round being ejected without problems. But it is possible that the round will detonate inside the vehicle, destroying the tank and sending shrapnel all over the place. For Ordnance, the problem isn't the tank being nuked, it's the shrapnel.
2. Remove the gun from the turret and move it to a safe place. Ordnance will put at least three shaped charges on the outside of the chamber. One aimed at the primer, one aimed at the propellant and the last one aimed at the shell itself. The whole barrel will then be covered with several tons of dirt and the charges detonated. Voila, another safe ending to a dangerous situation. The gun is properly ruined, but nobody is hurt (except maybe some feelings).
I'm assuming that the gunner knew how to handle firearms and various weapons. He had fired the gun before, he knows how it works. He might not have much experience with misfires, but he does know that he should wait a bit before opening the breech. But at this point, it's not a round you have in the gun. It's not a misfired round. It's not a nuisance. It's a faulty round.
It's an explosive. It intends to kill. And it intends to kill you. And it intends to kill you immediately.
Treat it as such. Don't touch anything. Sod off to a safe place. Call Ordnance.