exact mode of explosion caused by a short throw

[Sig_Dude]

I have read many times that not putting enough powder in a cartridge either on purpose or by accident can cause a fatal kaboom incident. I have tried to understand but just can't what the exact mode of explosion is. I would think that, assuming the bullet had just enough oomph to clear the muzzle that you'd be OK. I would also assume that if the bullet got stuck in the barrel, the gas would simple leak out the action or barrel-cylinder gap. I am aware that if you fire a squib followed by a real-deal you'd blow up your barrel.

So what *exactly* happens when you unknowingly fire a cartridge with not enough nitrocellulose?

[Krieger9]

Everything I've read about this is that it's quite a mystery but annectdotal evidence shows it to be a rare problem.

I'd love to hear better though too.

[Brian Pfleuger]

Two possibilities that I am aware of:

Both involve low charges of "slow" burning powders.

Not so much that there "isn't enough", it's that there isn't enough to properly fill the case and the flash from the primer can travel over the top of the entire charge, essentially igniting the entire charge all at once rather than allowing the flame to spread through the powder and create pressure "slowly" (slowly in terms of fractions of a millisecond).

The second possibility involves slow burning powder and a light bullet. The initial pressure increase can accelerate the bullet so quickly that the powder can't "keep up" as the bullet travels. The resulting pressure drop causes the powder to self-extinguish. The bullet not having pressure building behind it begins to slow down and the shock wave of still expanding gases behind it "catches up". The area directly behind the bullet is highly compressed by the shock wave and that high pressure area reignites the powder. This occurs down near the muzzle end where the gun is not built for chamber type pressure levels. The result is that the muzzle end of the gun explodes. This is what is referred to as "Secondary Explosive Effect".

[jmorris]

Detonation is the phenomena in question. When it happens in an engine one flame front is ignited by the sparkplug and another by so other means (carbon build up or just a hot spot and low octane fuel). When the two flame fronts collide the pressure is way above what you see under normal conditions. I have enough hunks of blown up race engines to prove that this can and does happen.

The theory is the same but as above the primer has to light the front and back of the charge and not the middle. I have never seen it or the loch ness monster but people say both exist. FWIW I don’t swim in loch ness or load small charges of slow powder in large capacity cases.

[totaldla]

It's a myth. It has never been proven. It has never been recreated in a lab setting to analyze. The case where somebody says "detonation" were simply double-charges.

[Sig_Dude]

So what if I want to make a real powder puff .38 load for my wife. If I use VV N310 (VERY fast) and a 125 grain bullet, would I be OK despite the fact that the case will not be full?

Also, do full cases generally burn the cleanest?

[Brian Pfleuger]

So what if I want to make a real powder puff .38 load for my wife. If I use VV N310 (VERY fast) and a 125 grain bullet, would I be OK despite the fact that the case will not be full?

Also, do full cases generally burn the cleanest?

Very fast powders due not suffer from this problem, whether it be real or imagined it is only a problem with slow(er) powders. I have used small charges of Unique in my rifle cartridges to fireform the cases to the chamber. They were only about 35% full.

The very fast powders burn so fast that in many instances the powder is completely burned before the bullet has moved even an inch, sometimes before it moves at all.

Case "fullness" doesn't determine clean burning. That is a property of the powder itself and also influenced by the peak pressure reached. Generally, if your load produces a pressure that is in the range of the powders design then it's going to burn as cleanly as that powder will burn, slightly more so as pressure increases, generally.

[Sig_Dude]

So I should, in theory, be able to go BELOW the minimum powder charge...I am going to try that. I guess if a bullet gets stuck I will know I have gone too far!

[Brian Pfleuger]

Try 2.5gr N310, maybe start a little higher. If you're careful, the worst that can happen is a squib. Just make sure you have a hole in the paper before pulling that trigger again.

[Sig_Dude]

That sounds like a load that will bruise the bad guy Perfect for target, as long as there is some degree of accuracy.

Note that I am using jacketed bullets...Hornady and Speer.

[Brian Pfleuger]

I ran that load through QuickLoad using (I think) the Speer 125gr FMJ. It says 705fps, 138 ft/lb, with a 5" barrel. Essentially a 22LR. It's probably a little optimistic since QuickLoad doesn't account for the pressure loss at the cylinder gap. You might want to start at 3.0

[riverwalker76]

I suppose you are talking about what I refer to as the 'Railway Gun Phenomenon'. Back in the early days of WWI Germany their Railway Guns kept blowing barrels and they couldn't figure out why. They were using the wrong speed powders for the length of the barrel and weight of projectile, and once this was addressed the problem stopped. The pressures were building up so fast inside the barrel between the point of ignition and the point of projectile exit that it stressed the steel beyond it's designed strength. The result was a thousand tiny pieces of shrapnel covering the surrounding landscape after firing. Not to mention the loss of life surrounding the railway gun due to the explosion.

If you follow load data and stick to designed bullets and powders for your rifle / pistol. You will not have this problem unless you grossly overload or underload your cartridges. Underloading will more than likely cause a 'squib' load and will not reach the pressures at which your chamber and barrel are designed.

[Sig_Dude]

Especially since I will be firing these out of a .357. I should be very safe, I would ASSume.

[Edward429451]

Very fast powders due not suffer from this problem,

That's incorrect. It was a 38 Special with a teeny charge of Bullseye that started all this detonation stuff. Hercules tried and tried and could not recreate the phenomenon. I've never heard of it happening with slow powders, only fast powders.

[jmorris]

If you want very light 38/357 loads go get some from the SASS guys. I have read of some that wouldn't go through 3/8" plywood.

[wogpotter]

Here's the theory.
Modern powders are "progressive burning" in that the burn rate varies with the pressure in the container.

You have a very light load. It doesn't burn at a "normal" rate but "fizzles" just like it would if you sprinkled it on the ground & threw a match into it.

Eventually it builds up just enough pressure to pop the bullet out of the case & into the barrel leade. At this point the pressure drops a lot as the volume increases.
Now you have an air space that is larger than normal & a charge that is even lower still than normal still "fizzing", but there is more of it fizzing, just like a fire increases the amount of fuel if left to burn up.

Now the pressure climbs, the burn rate increases dramatically, way pas the point where the bullet would normally start running down the bore, balancing burn rate against gas production in a controlled high speed fire.

Unfortunately the bullet is jammed in the start of the rifling with no "running start" to engrave it so it can't move, the pressure climbs, the burn rate climbs & eventually it accelerates to the point where there is a detonation.
Voila one "Kaboom".

As to it "never having happened" I remember seeing pictures of revolvers blown apart by supposed "low squib loads" so I don't know if this mechanism is the exact detailled cause of the event, bu I do know very well I don't wanna be there if the event occurs, no matter how it is caused.

[Sig_Dude]

Has anybody ever been killed by a low load in a pistol? Or is it usually cuts and burns on the hand(s)?

[Unclenick]

I don't know of detonation happening in pistols at all. The cases are small. The old "Bullseye surprise" in .38 Special proved to be caused by double charges the loader somehow missed. But when a gun or cartridge bursts for any reason, it is potentially fatal and some severe injuries have occurred. The only fatality I am aware of for sure was a soldier killed by an open breech slamfire in a machine gun that broke pieces off the bolt. The unfortunate soldier was to the right the gun and exposed to fragment from the ejection port. I have heard of a few others over the years due to barrel obstructions. A moderator on another board lost his left hand to a shotgun burst this last year, and the cause is still under investigation.

The usual detonation story is different from most burst causes. It usually comes from someone trying to make subsonic catsneeze loads in a big case where they have over 80% empty space. This most often is possible in rifle cases and is most often with a slow rifle powder mainly because that is the powder the rifle loader had on hand and maybe because the deterrent coatings help create detonation conditions described below. However, the Finnish Gunwriter's site describes a .308 rifle that was blown up by a charge of 3.1 grains of N320 (about 88% empty space), which is a pistol powder between Bullseye and Unique in burning speed, but closer to Bullseye. A simple double charge won't explain that event because you would need seven times that charge of N320 just to come up to normal .308 pressure, much less to do any damage.

Since detonation depends on ignition at the rate of a shock wave traveling through a contiguous explosive mass or its approximation (as by very large amounts of powder), suggests that the detonating charge has to fuse into a lump first. Once you have hot fused powder, if it is at the correct temperature, simple ignition can initiate detonation because heat has already raised the potential energy in the powder to the point it is on the edge of spontaneous ignition and explosive gases evolved by the heat are at its surface. In that situation any ignition starting point could create pressure fast enough to grow into a shock wave that triggers detonation of the rest of the mass. That seems to be what set off the Texas City port explosion of ammonium nitrate in 1947.

This set of conditions is not easy to create. It requires first that the case let the charge be far enough from the primer that the sparks miss hitting it, instead simply creating enough heat to fuse it. It is an unlikely event, but if you look at photos of how flash hole burrs can direct the primer flame and sparks to just one side of a case, it's not impossible that it occurs. Another possible scenario is the powder starts out burning, then extinguishes, but is still fused by the heat (fusing itself absorbs heat of phase change, and may be instrumental in the extinguishing mechanism). Perhaps it has to divide into two piles, one of which burns enough to make heat to fuse the other without igniting it until it is mostly fused? Nobody knows, but all the explanations involve the stars lining up just right and very improbably.

[totaldla]

Hmmm, well I don't know anything about N320, but I seriously doubt that 3.1grains of it can produce enough gas volume to raise pressures in a .308 chamber beyond the yield point of steel.

So I can only assume that the powder, which normally converts to gas at a rate that produces pressure wave traveling at about 7000fps, somehow changed much faster, (about 18000fps or so), exceeding the mechanical properties of steel causing it to disintegrate. I'm not an ME, so I don't remember the name, but it is fundamentally akin to flexing a Big Hunk candy bar too fast - it snaps.

Now I know ANFO does this quite readily (thus the 1947 BOOM). But I didn't know it was possible to take a wood chip and oxidizer and do the same thing. I though the fundamental combustion process was just too slow regardless of the pressure.

I would think a barrel would have to fracture, (disintegrate) and not burst for it to be a "detonation" event.

[briandg]

I need to add in here that smokeless powder cannot detonate. Detonation means for a material to decompose into heat and gas almost instantaneously, and at the speed of a shock wave.

In truth, none of the nitrocellulose products can actually do that. They can only burn.

the report above that referred to a bullet trapped in the leads, and the charge going through the final phase of burning is one I've heard of before, and makes the most sense to me. If a charge ignites, fails to completely burn, burns off all the coatings and deterrents, and finally goes into the final phase of rapid burning in a small space with full pressure, and no way of instantly opening new space, it can create a pressure spike that could cause catastrophic failure.

Engineers in the powder industry have calculated that there isn't a way to force a detonation of powder. Neither heat, pressure, or any known phenomenon can cause instantaneous combustion. They agree that sometimes there are anomalies.

[riverwalker76]

X2 on what Unclenick said.

He will never steer you wrong, and he's full of good advice.

[wncchester]

Massive chamber pressures with small charges seems counter intuititve. What's not appearant is that powders, especially slow powders, don't burn at a linear rate. The burn rate changes dramatically and quickly as pressure increases. In extreme instances, increased resistance to bullet movement can stop it long enough for pressues to skyrocket in a thousanth of a second.

The process is thought to be the bullet moves forward under the pressure of the primer but the light powder burn intially has too little pressure to continue to move it. If the bullet stops, or even slows very much, the remaining powder will suddenly burn MUCH faster than normal. Pressure can increase so rapidly the intertia and resistance of the lodged bullet can't move and expand the chamber volume fast enough, so things start to seperate.

It's rare in handguns EXCEPT with certain powders such a H-110, W-296.

[F. Guffey]

We talk about neck being too tight to release a bullet, powder that is position sensitive, and the most vague one, detonation, something like two flame fronts hitting together etc., etc., etc., Then someone post pictures of his Waeatherby 300 Win mag that has been rendered scrap and claims it should have never happened, simple rational, the rifle was served a steady diet of reduced loads, next reply? "You 'mus-ta' double charged one", after that we go on repeating the same old answers. Meets and or exceeds, sudden shock?

Just for the heck of it prime a case, seat a bullet (no powder) then chamber the case and fire it, the primer will drive the bullet out of the case neck and into the throat, and there is where it lodges, wedges, stops. I suppose someone could argue the sequence of events without giving a thought to adding powder and it's effect if the primer moves the bullet first before igniting the it. Then add another consideration, a reduced load will allow the powder to lay in the bottom of the case, this allows the flame of the primer to pass over the top of the powder causing a larger surface area of powder to ignite, the event happens in milliseconds, I choose not to jam a bullet in the throat before the powder starts to burn, I like the ideal of a running start.

For those that think it is a good ideal and favor the Sargent's exemptions to disease can first jam a bullet into the throat THEN add powder, case and primer with various amounts of powder, fast and slow, as for me I believe reduced loads are cute, I form first then fire to get once fired cases and when forced to fire form I do it once, as Seaweed said of my fire forming, "that sounds like some scary and risky stuff" my fire forming loads were at or beyond maximum for the 30 Gibbs after forming. Out of respect for the Arizona gunsmith I called Hodgdon to explain my thinking and informed them of his concern/caution, Hodgdon said my method will work BUT do not use that load as a starting load when developing loads for that chamber, because of the time factor, the events in milliseconds.

F. Guffey

[totaldla]

Seesh I'm getting old - I just read the guy's work and forgot his name. Anyways he tested all sort of obstructions in 1903's to see what was dangerous to the shooter. And he found a lot of ways to bulge/ring barrels and he found some sure-fire ways to burst them. BTW, he never found a way to blow a bolt back into a shooter's face.

I believe the key in the discussion about detonation is the mechanical failure mode. If a barrel/chamber bursts - it is NOT detonation. Detonation would cause it to disintegrate and fragment. I've seen lots of double-charges, squib obstructions, etc. and they all caused bursting.

What happened to the German rail guns sounds like detonation, but I don't know how those barrels were made or how big the pieces were.

I have never heard of H110/W296 causing anything other than squibs.

As I said in the beginning, I believe detonation is a myth, a gun-forum old-wives tale, that makes its rounds periodically.

[F. Guffey]

http://www.odcmp.org/1101/can.pdf

In the old days Enfield started development of a rifle that became the P14/M1917 with failure built in, the chamber was similar to the 280 Ross, they were hard headed, they insisted on using a powder that would cook off before the trigger was pulled, then we, in an attempt to save money, used reduced loads in the 03 for target practice, that effort did not last long because of the marginal safe design of the 03, meets and or exceeds and sudden shock, then there was the slide and glide attempt, by accident and or design cases and or bullets were lubed to prevent fowling of the barrel, again meets and or exceeds or sudden shock, the practice was abandoned, no one knew if the sudden shock created in the next 03 was going to cause the receiver to swarm, then there are the accounts of receivers being hit with hammers, it is believed the sudden shock shattered the receiver. At best the receiver of the 03 was marginal.
F. Guffey