Pressure question

[Bob with the 66]

I would like to learn more about case pressure, I was told that if I move the bullet further out of the case, not changing the charge amount, the case pressure increases.

Then if I more the bullet further into the case, not changing the charge amount, is it true that case pressure decreases?

Learning as I go, Bob

 

[seymour fish]

Bob, Imagine a U-shaped curve, where for a given powder charge seating into the lands raises pressure by virtue of a higher starting pressure to move the bullet and where seating the bullet deep into the case raises pressure by virtue of a smaller usable case capacity. Seymour

 

[Bob with the 66]

So if I more the bullet .010" out of the case or move it .010" into the case the pressure is increased the same amount?

 

[CatShooter]

I would like to learn more about case pressure, I was told that if I move the bullet further out of the case, not changing the charge amount, the case pressure increases.

Then if I more the bullet further into the case, not changing the charge amount, is it true that case pressure decreases?

Learning as I go, Bob

Neither is true. The pressure is determined by the amount of powder and the "burn space" - the burn space is determined by the place that the bullet does not want to move forward - the point of resistance) - that place is the throat/leade.

The amount the bullet is in the case means almost nothing, be cause the bullet leaves the case at very low pressures, until it hits the leade. Then the pressures start to rise, cuz that is where the resistance is.

Roy Weatherby got around pressure problems by moving the throat/leade forward, to lower the pressures.

The peak pressures are not reached until the bullet is well into the barrel.

 

[Bob with the 66]

Then where the bullet sits in the neck has no play in creating pressure.

So pressure variance is only generated from the size of powder charge and primer choice?

I'm trying to understand, Bob

 

[CatShooter]

I will send you a PM...

... sent!

 

[Jeff Conover]

CatShooter is on track here...The pressure is determined by the amount of powder and the "burn space" - the burn space is determined by the place that the bullet does not want to move forward - the point of resistance) - that place is the throat/leade.

The amount the bullet is in the case means almost nothing, be cause the bullet leaves the case at very low pressures, until it hits the leade. Then the pressures start to rise, cuz that is where the resistance is.

A cartridge does not reach peak pressure until the bullet is sealed in the barrel about 3.5 inch down the bore. When the cartridge is fired the case expands and seals at the case neck first and new gas escapes around the bullet moving down the bore at about 4,700 fps. When you get the bullet pass 4 inch of the barrel the pressure curve is on the downward slope. Seat the bullet deeper in the case and more new gas escapes around the bullet before it seals in the barrel. The bullet is acting like a check valve in the throat releasing the new gas. You can see this by using the same powder charge and changing seating depth on the bullet. In my dasher .010 set depth with XBR-8208 will change the velocity 15 fps. Your mileage will vary depending on throat condition, bullet design, and case capacity. I have done this testing on my own and at Olin-Winchester's ballistic department.

Moving the bullet in and out of the case and measuring pressure require precise material and equipment. Case volume, neck tension, primer quality, powder position, bore condition, and temperature all affect the outcome.

Jeff Conover
Dutch Hollow Gunsmith
DoD Ammunition Quality Representative

 

[Bob with the 66]

Thanks all for the explanations, I understand now.

At least the basics.

Bob,