Under Charge and Overpressure

[Curmudgeon]

Can someone point me in a good direction to understand why excess pressure happens when a load isn't fully charged. I have read some explanations but still don't understand. This isn't a question about wrong powders but about a less than full charge resulting in dangerous pressure. Thanks

 

[BoydAllen]

Google "secondary explosion effect" and do a lot of reading, not just the first thing that you see, but pretty much all of it.

 

[CatShooter]

Can someone point me in a good direction to understand why excess pressure happens when a load isn't fully charged. I have read some explanations but still don't understand. This isn't a question about wrong powders but about a less than full charge resulting in dangerous pressure. Thanks

As to whether it is a myth, is hard to say. Along time ago, P. O. Ackley said it happened. While Ol' Parker Ackley was a good gunsmith, and a fair wildcat cartridge designer... he was also a hellova showman and bs artist. He was prone to "be creative" at times.

That being said... It IS possible, but as far as anyone knows, it has not been reproduced deliberately in a laboratory.

But "IF" it happened, it happens like this.

The early stage of firing a cartridge is very complex... and it is a balance of many things in order to make it work.

When a bullet is fired, it leaves the case neck and jams into the throat/leade of the rifling and for a minuscule moment, it slows, while the gas pressure is building up high enough to force it into the rifling and on out the barrel.

This is a critical time, because if the bullet stops, it is extremely hard to get it going again, because it is stuck in the throat, and something called "sticksion" (stik-shun) takes over and then it takes a humongous force to get it going again. Sticksion was discovered by John Bell Blish, when he was studying large artillery. It is the same phenomenon that causes the walls of a cartridge to stick to the walls of a chamber when it is fired (leading to head separations).

http://en.wikipedia.org/wiki/Blish_lock

When a bullet is in the case and the powder is first lit, normally this stage is the time that the powder also gives off the most gas, because the particles have the largest burning surface area... but since the bullet is barely moving, this is the time when you want a limited/controlled amount of gas given off, so you can start the bullet into the rifling... then, once the bullet is moving faster, and there is a larger volume of space in the bore to fill... then you want more gas given off.

Normally, smokeless powder works the opposite way - it gives off the most gas in the beginning, and then gives off less and less gas until powder is gone.

So, powder has a coating called a "deterrent", which slows the early burning stage to let the bullet get moving. Then, when the flame consumes the deterrent, the burning gets faster so it can generate gas at a faster rate to keep the bullet accelerating out the barrel.

So, the normal sequence is... the primer "lights" the powder, and the bullet gets started... it moves into the throat/leade and the powder gives it slow, steady push past this early resistance, and the bullet starts moving in the rifling... the deterrent is consumed and the powder starts burning much faster, but there is more space for the gas in the bore and the bullet is now moving fast, and is on it's way.

Now, this scenario is possible when a large case, long bullet, and light load of slow burning powder is fired....

With a large case, and a very long bullet with a long bearing surface, and a small amount of slow burning powder - the primer lights the powder, but the pressure in the case is low - the powder pushes the bullet into the throat/leade, but the small powder charge has not created enough gas to keep the bullet moving and it stops.

Notw the bullet is stuck in the throat/leade, and the small amount of powder burns through the deterrent coating and now starts to burn faster... and the bullet is still stuck. The pressure keeps rising and the powder keeps burning faster and faster (because that is what "progressive burning" powders do), and even if the bullet breaks free, it is now moving slowly up the first few inches of the barrel, while the powder is burning fast so it can generate the volume of gas that it should, IF the bullet was accelerating half way out of the barrel, the way it should have been.

So stuff lets go.

Some people have complained of bolts being hard to open with light loads of slow powder... but no one has blown up a gun that can be traced to the Blish effect.

However, it IS possible, and there is no reason to do it... you can use faster powders and lighter bullets for fire forming or blowing out case shoulders.

(These is no secondary explosion effect, because there is no primary explosion - powder burns, ie, deflagrates, it does not explode.)

 

[stubbicatt]

Man. I'm not going to do this justice. But here goes.

Years ago I bought a pressure testing kit which works with a transducer glued to the barrel. Among the programs and other documentation that came with that kit I remember reading about a phenomenon of powder which is slow for a given cartridge, which was revealed by using this kit. IIRC, with the slow for cartridge powder, there is a secondary pressure spike when the bullet is a long way down the barrel. I don't remember the explanation.

I wish I remembered more. Nonetheless, with rifles I tend to stick with faster burning powders appropriate for that cartridge in effort to avoid this from happening. Sorry I don't remember more.

 

[bheadboy]

Catshooter has the answer,

a sumwhat simple version from my experience and reading, is if you use a significant reduced load of a slow burning powder, there is the POSSIBILITY of a staggered pressure curve resulting from slow ignitian vs bullet movement the accelerated ignitian causing a pressure spike.

never had it, read about it, must happen - use faster powders in reduced loads.

Bob

 

[lpreddick]

CatShooter: i like your explaination. if the bullet is well into the lands(.020-.025 thous) will this phenomenon be less likely?

 

[gunsandgunsmithing]

A good read all together but this subject, specifically, begins about half way down the page below.
http://www.shootingsoftware.com/barrel.htm

 

[CatShooter]

CatShooter: i like your explaination. if the bullet is well into the lands(.020-.025 thous) will this phenomenon be less likely?

No matter how far it is into the lands, if the initial pressure is not enough to keep it moving, you gots BIG problems.

 

[Shynloco]

IMHO, CatShooter provides an excellent technical and detailed explanation of what "CAN" occur when using a light load. I have a good shooting friend with many years of experience (50+) as a reloader and shooter who experienced the reality of what a light load CAN do. A few years ago, he decided to test an almost brand new Savage model 12 .223 by going a few grs UNDER the minimum recommended load pushing a 52 gr Sierra using Varget. Essentially, it is suspected that there was more empty space in the top half of the loaded cartridge when it was loaded into the chamber. The ambient temperatures was 90+ at the range. Upon pulling the trigger, there was one helluva explosion and the rifle was blown into two separate pieces, with a shattered wooden stock and a severely permanently damaged receiver and welded bolt. I'd seen pictures of top straps blown off handguns before (cylinder out of timing). But this was the first time I'd ever heard of a light load blow a rifle into pieces. My friend was lucky because he ended up with only a slight facial cuts. From that incident, I took away the knowledge that what CatShooter describes CAN happen when someone gets too cute testing a light load.

Alex

 

[BoydAllen]

As far as the search term that I gave goes, I was just giving a route to information. IMO pretty much all of the explanations are guesswork of one sort or another. Pressure spikes can be measured and correlated to types of loads, but as to the mechanism, I believe that that they are guesses, however well educated.

 

[Lapua40X]

... less than full charge resulting in dangerous pressure....

Perhaps I've missed something here but, what does "less than fully charged" mean in this context? If we're looking at a range of suggested charges in a reloading manual the loads listed, which are based on specific bullets with specific COAL (generally not jammed loads), can often develop dangerous pressures well before the maximums within the published range.
I read about long range 6BR shooters using 30 grains of Varget behind their 108 grain bullets . My 6BR says "don't push your luck" at 28 grains of Varget with a .020 jump. Be careful not to get hung up on the dangerous practice of relying on loads that work for other shooters to work with your rifle. There are more than a few shooters (former shooters) who have half their face missing because they reloaded using that style.

 

[BoydAllen]

The charges in question would be below those listed in a manual. If you do the search that I suggested in my first post, and do some reading, you will find out pretty much anything that you want to know.

 

[bigedp51]

Can someone point me in a good direction to understand why excess pressure happens when a load isn't fully charged. I have read some explanations but still don't understand. This isn't a question about wrong powders but about a less than full charge resulting in dangerous pressure. Thanks

When your not fully charged with a solid propellant excess gasses can build up with explosive effects.



You can also email Winchester and ask why they don't make 785 powder anymore. Or except the fact that light charges of slow burning powders can detonate as you are warned in the reloading data.

The question you are asking was determined under testing by the manufactures of the powder. It does happen and the reason can get fuzzy depending on who answers the question.

As an example in the Winchester reloading pamphlet:

1. 32 Winchester Special, 170 grain bullet, 748 powder, 46 grains, 32,500 cup.

2. 8mm Mauser, 170 grain bullet, 748* powder, 46 grains, 37,000 cup.

If the load has an asterisk* you are warned not to change components or reduce powder more than 10% or it can cause dangerous pressures. The major difference is the case capacity is larger on the 8mm Mauser with the same charge of powder. (excess air space in the case)

 

[savagedasher]

It happens when a large case is used with a fast burning powder. You get secondary ignition. When you use 20 gr of powder in a case that holds 75g you must us a filler. That keeps the powder charge next to the primer not at the bullet end. Good Shooting Larry

 

[bigedp51]

It happens when a large case is used with a fast burning powder. You get secondary ignition. When you use 20 gr of powder in a case that holds 75g you must us a filler. That keeps the powder charge next to the primer not at the bullet end. Good Shooting Larry

savagedasher, I'm hoping that statement was a brain fart...............

I have been loading and shooting light charges of fast burning powder in my .44 magnum for the last three weeks. That would be 7 grains of Titegroup instead of 24 grains of 296 powder.

The problem is light charges of "SLOWER" burning powder in larger volume cases hitting critical mass.

The reason a filler is used with light charges is to keep the cartridge case from being position sensitive and getting uniform ignition and pressures.

 

[savagedasher]

What I was saying a case that is 40% full of fast powder. That is tipped down when loading and the powder is at he bullet end you can get secondary explosion. P O Ackley covers that in his books.
Anybody that does or is going to reload I feel should read both his books. Good Shooting Larry

 

[expiper]

Dear crumedegon,,,,OP....I have been loading reduced velocity ( not reduced pressure ) loads since early 60's when I started,,, I was fortunate enuff to be schooled by one of the smartest shooters/loaders in the country,,,,besides all of the accuracy and hi-vel loading knowledge he shared with me he also was a nut on reduced loads,,,,he taught me how to make ammo for most common deer hunting cals. that culd be used for turkey hunting etc,,,,I cant give any data or info on the internet because of all the moderators ,internet shooters,and nay sayers that dont understand it or apreciate it,,,,but any how....several yrs ago (80's ) a man that I worked with asked about my methods and we didnt go into great detail,,,,he asked about the powder charge wt,,,and I told him most were between 10-30 grains and it depended on the cal, and intended vel.....and that it was pistol/shotgun powders that I used,,,well you know how a story can change once it gets told a cupla times,,,,by the time it go to the thrd or 4th person it was ,,,just use 10-30 grs of powder,,,,,,and a fellow did that in a mag. of som sort using his regular powder (prolly 4831 or its equal)and ,,,,IT DID BLOW THE GUN UP,,,,he had minor burns and bruises ,,,thank God,,,,
any how ,,pressure excursions can happen,,,,dont do it or load it unless you know what ur doing,,,,Roger

 

[savagedasher]

P O Ackley books are listed on E . Arthur brown reloading. I feel they are a must read for anybody doing or is going to reload. It show pictures of what happens when you under charge.
Good Shooting Larry

 

[bigedp51]

What I was saying a case that is 40% full of fast powder. That is tipped down when loading and the powder is at he bullet end you can get secondary explosion. P O Ackley covers that in his books.
Anybody that does or is going to reload I feel should read both his books. Good Shooting Larry

I'm glad you clarified that and said 40% full, my 7 grains of Titegroup in a .44 Magnum case is only 33% full and I'm 63, still in one piece, with all my fingers.

I guess my .40 S&W is a real safe load at 43% and 3.7 grains of Titegroup. The only one who was injured and died was the paper target. : I think it was heart failure from being shot with a 165 grain plated bullet at only 850 fps.



I won't say anything about my .38 special, 148 grain HBWC, 3 grain of Bullseye at 26% of case capacity, because I do not want to frighten the children who may be reading this. wink, wink

P.S. It doesn't matter what P.O. Ackley said, I'm only shooting the un-improved cartridges.

 

[savagedasher]

When did a 44 mag get to be a large case. I think you have missed the point he was asking.
Good Shooting Larry