Under Charge and Overpressure

[Prostockstan]

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

 

[Prostockstan]

Can someone tell me if this is possibly the problem I'm having. I used a 300 win mag tactical match reamer for the chambering. This reamer provides a much longer throat area. Thus My OAL with 208 ELDs is set at 3.620 leaving me a .020 jump to the lands with 72.6 g of reloader 22 The bullet is out of the cartridge much farther compared to a SAMMI standard chamber. Does the excess area inside the cartridge require a higher charge because the bullet is much farther out? The primer has two small burn holes around the outside edge of primer and also burned two small pockets in the bolt face. I will be re-machining the bolt face and will have to reface the barrel shoulder to make up the difference. I would like to know if anyone has any load data for the PTG 300 win mag tactical match reamer. I do not want to burn up this rifle again.

Thanks: Stan

 

[savagedasher]

It normally is from secondary powder burn.
When the primer starts the ignition there isn't powder for it to burn . If you have shot muzzloader much it is what they call hang fire . With a muzzleloader the problem isn't a serious as it is with center fire because the bullet is smaller the bore. But when they use smokeless powder when breach loading they use a filler to keep the powder at the primer to stop secondary burn. Or they blow the gun up . I think the over pressure signs it incomplete powder burn . Or secondary pressure peak . Larry

 

[Ggmac]

Sent pm to the OP

 

[ireload2]

There are always people talking about this and none of them have ever experienced it.
I have had this happen while shooting the same loads through 2 identical rifles during the same trip to the range. They were both M1891 Argentine Mausers in 7.65 Mauser. One had seen a little use and one was like mint new.
The powder was AA2495 and the starting load I used was 39 grains which I had arrived at by calling AA and by looking at AA loads for similar rounds.
To make the story short the like new rifle blew 2 primer pockets about .015 over size. The other rifle did not have any problems with the same loads. Starting loads fired in both rifles sooted the outside of the case. Even the loads with the blown primers sooted the outside of the case. I noticed the bolt of the newer rifle had a soft firing pin fall and stripped it to find it full of cosmoline. I cleaned up the bolt and fired it with no more problems. I even swapped bolts and fired the new rifle with no problems.
My theory was I had poor ignition with the cosmoline filled bolt. I eventually finished working up the loads to 43 grains without any more problems.

I suspect that the soft firing pin fall produced poor ignition with the light starting load. I think I got a slip stick episode when the bullet moved and then stuck momentarily causing the pressure spike.

 

[Bill Conrad]

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.)

Just a quick comment. What Parker Ackley warns about in his Volume-1... about reduced loads... is that they can distribute themselves along the cartridge case and thereby allow multiple ignitions to occur. A filler (dacron, etc.) is needed to make sure the powder is up against the primer flash hole.

Bill

 

[Dusty Stevens]