44man
04-11-2007, 10:26 AM
I was going through my old handloader magazines and came across the article in the June, 1997 issue. It seems to be the best explanation yet.
A 6.5 Swedish blew up but the barrel was ok so they mounted it in a pressure receiver and tests made with piezoelectric pressure system. The same ammo was used for a 10 round test. No. 1 showed 48,820 psi. Pressure increased with each shot and at round 8 it was 82,120 psi. That stopped the test because of danger.
The pressure curve started to rise, then dropped to 3,800 psi for a millisecond, then climbed out of sight. Engineers said it takes at least 5,000 psi to keep a bullet moving so this bullet stopped for a millisecond and became a bore obstruction.
Another interesting thing was the velocity between the low pressure shot and the high one was only 274 fps.
They estimate it was the long lead or throat erosion and the small amount of pressure needed to eject the bullet from the case increased the case volume so the rate of gas production could not keep up, then it finally did with a stopped bullet.
He mentions that sometimes we get lucky and the bullet starts to move to relieve the pressure and that a soft boolit can save the gun.
One of the recent posts here about the powder not burning in a .458 with a cast boolit really alerted me. My take is that the softer boolit moved far enough into the bore that the primer fire scorched the powder but the fire went out. IMHO, the primer pressure blew the boolit and powder too far from the fire. I would not shoot those loads again unless neck tension is increased or the boolit is jammed into the rifling to hold it in place. A hotter primer can help but does not solve what could be a dangerous situation. Some primers have a lot of pressure without the heat needed and can make it worse. Either way, if the boolit is not held in place until the powder can increase gas production smoothly, SD's and ES's and pressures from shot to shot can vary so wildly, accuracy is non-existant.
A loose, greased paper patched boolit with a slow smokeless powder SOUNDS DANGEROUS to me.
This is a common problem in a big bore revolver with a lack of neck tension where the boolit is blown into the forcing cone before the powder burns. THE SAVING GRACE IS THE CYLINDER GAP OR WE COULD BLOW UP REVOLVERS! Accuracy goes out the window. It is the reason I want to see the base of the boolit and grease grooves on the brass of a loaded round, the reason I use only Hornady dies too.
On occasion I find a flatter primer from my revolvers. I figure this round had less tension and the boolit came out too soon, making a tiny case of S.E.E. I found that this round will hit lower on the target because of increased velocity. This makes me appreciate the gas relief of the gap. Doing this in a rifle without the cylinder gap is asking for trouble.
My suggestion for the .458 is to use a faster powder, increase neck tension, use a hot primer with low pressure, engage the rifling if possible. One other thing to watch is to not seat a boolit out so far that there is not enough in the case to be held snug. A soft boolit can still be driven into the bore by the primer alone. There is a balance to be found with neck tension and a boolit into the lands.
A 6.5 Swedish blew up but the barrel was ok so they mounted it in a pressure receiver and tests made with piezoelectric pressure system. The same ammo was used for a 10 round test. No. 1 showed 48,820 psi. Pressure increased with each shot and at round 8 it was 82,120 psi. That stopped the test because of danger.
The pressure curve started to rise, then dropped to 3,800 psi for a millisecond, then climbed out of sight. Engineers said it takes at least 5,000 psi to keep a bullet moving so this bullet stopped for a millisecond and became a bore obstruction.
Another interesting thing was the velocity between the low pressure shot and the high one was only 274 fps.
They estimate it was the long lead or throat erosion and the small amount of pressure needed to eject the bullet from the case increased the case volume so the rate of gas production could not keep up, then it finally did with a stopped bullet.
He mentions that sometimes we get lucky and the bullet starts to move to relieve the pressure and that a soft boolit can save the gun.
One of the recent posts here about the powder not burning in a .458 with a cast boolit really alerted me. My take is that the softer boolit moved far enough into the bore that the primer fire scorched the powder but the fire went out. IMHO, the primer pressure blew the boolit and powder too far from the fire. I would not shoot those loads again unless neck tension is increased or the boolit is jammed into the rifling to hold it in place. A hotter primer can help but does not solve what could be a dangerous situation. Some primers have a lot of pressure without the heat needed and can make it worse. Either way, if the boolit is not held in place until the powder can increase gas production smoothly, SD's and ES's and pressures from shot to shot can vary so wildly, accuracy is non-existant.
A loose, greased paper patched boolit with a slow smokeless powder SOUNDS DANGEROUS to me.
This is a common problem in a big bore revolver with a lack of neck tension where the boolit is blown into the forcing cone before the powder burns. THE SAVING GRACE IS THE CYLINDER GAP OR WE COULD BLOW UP REVOLVERS! Accuracy goes out the window. It is the reason I want to see the base of the boolit and grease grooves on the brass of a loaded round, the reason I use only Hornady dies too.
On occasion I find a flatter primer from my revolvers. I figure this round had less tension and the boolit came out too soon, making a tiny case of S.E.E. I found that this round will hit lower on the target because of increased velocity. This makes me appreciate the gas relief of the gap. Doing this in a rifle without the cylinder gap is asking for trouble.
My suggestion for the .458 is to use a faster powder, increase neck tension, use a hot primer with low pressure, engage the rifling if possible. One other thing to watch is to not seat a boolit out so far that there is not enough in the case to be held snug. A soft boolit can still be driven into the bore by the primer alone. There is a balance to be found with neck tension and a boolit into the lands.