Internal ballistics : case capacity effect on pressure/velocity

[ebf]

Question for the internal ballistics gurus...

For argument sake, let's say you have 2 batches of brass. One batch has water capacity of 55 gr, the other batch 56 gr.

If everything else stays the same (same projectile weight, powder charge, seating depth etc), which batch of brass would result in higher muzzle velocity and why ? I'm keen to try and understand the physics behind this.

 

[mikecr]

Same charge in smaller volume = higher pressure, higher MV

 

[ebf]

Thanks mikecr, can you point me at some law of physics where there is a formula to prove that pls ?

On the Berger blog, there is a discussion about seating projectiles further out, and the resulting larger internal volume in the case causing slower MV. There might be some other factors involved...

http://www.bergerbullets.com/effects-of-cartridge-over-all-length-coal-and-cartridge-base-to-ogive-cbto-part-1/

The primary effect of loading a cartridge long is that it leaves more internal volume inside the cartridge. This extra internal volume has a well known effect; for a given powder charge, there will be less pressure and less velocity produced because of the extra empty space. Another way to look at this is you have to use more powder to achieve the same pressure and velocity when the bullet is seated out long.

And then there is this :

http://www.frfrogspad.com/intballi.htm

By using a cartridge case with greater capacity we provide for more chamber volume with the same bore volume, thus the expansion ratio (see definitions above) becomes less. As a result, the powder gases have been through less expansion by the time they reach the muzzle and muzzle pressure is higher. As the muzzle pressure is higher, the average pressure along the bore is higher and the bullet has a higher velocity. There is a trade off for this. Because the muzzle pressure is higher, more energy is carried off into the air unused (as flash and muzzle blast) and thus the efficiency of the load (the getting of energy out of the powder charge) is less.

getting more and more confused ;D

 

[skiutah02]

Same charge in smaller volume = higher pressure, higher MV

On the Berger blog, there is a discussion about seating projectiles further out, and the resulting larger internal volume in the case causing slower MV. There might be some other factors involved, but it seems to say the opposite ?

http://www.bergerbullets.com/effects-of-cartridge-over-all-length-coal-and-cartridge-base-to-ogive-cbto-part-1/

The primary effect of loading a cartridge long is that it leaves more internal volume inside the cartridge. This extra internal volume has a well known effect; for a given powder charge, there will be less pressure and less velocity produced because of the extra empty space. Another way to look at this is you have to use more powder to achieve the same pressure and velocity when the bullet is seated out long

No need to get too confused, these are in agreement. Drew

 

[lpreddick]

Boyle's Law is the physics behind this question. Boyle's law states that as volumn increases, the pressure of a gas DECREASES...all other variables staying the same. a larger case containing the same powder as a smaller case will generate LESS pressure and subsequently less velocity.

 

[seymour fish]

EBF, Imagine a U-shaped curve. Seymour

 

[CharlieNC]

PV = nRT

 

[DWalt]

EBF, Imagine a U-shaped curve. Seymour

I have seen that graph of a U-shaped (actually more of a V-shape with a rounded bottom) pressure vs. bullet jump curve in several places. What it basically says is that at some critical "Jump" distance (bullet to rifling), all other things being equal, the peak chamber pressure is minimized. Beyond that, either increasing or decreasing the "Jump" will increase the peak chamber pressure, but for different reasons.

 

[Canadian bushman]

Same concept as using dished pistons to lower compression before supercharging.

Creating more room for the stuff that goes boom.