# Pressure Increase with Diameter Decrease?

1. Feb 2, 2012

### Wabatuckian

Hello,

I am a firearms enthusiast. In my collection is a Gewehr 88/05. This was Germany's first smokeless issue rifle and it originally used a smaller bore than is used today in the 8mm.

It was converted in 1905 to take a 0.323" bullet instead of a 0.318" bullet. The leade (free expansion space in front of the chamber) was increased in diameter to make it supposedly safe to fire with 0.323" bullets as the bullet swaged down to the 0.318" diameter.

I have had folks tell me this high-speed swaging is a dangerous practice. Something about that answer doesn't feel right. I've had plenty of physics in college, but that was over 12 years ago now and I simply don't remember the physics to solve this problem. My dad, an electrical engineer, had his high-speed fluid dynamics etc even longer ago, so I'm a bit stuck.

I'm looking at it like this:

Assume a max operating pressure of 45,000psi with a 37,500psi optimal pressure. Assume a 150 grain projectile with a free acceleration space of 1-1/3" before swaging begins to occur.

Assume a control barrel (pipe) of 0.323" diameter and 33,000psi pressure inside a said barrel.

How would this change if the barrel were reduced to 0.318"?

I am thinking the change would be negligible as there would be a pressure spike during the swaging process, but nothing extreme. After the swaging process, the pressures should drop back down as the projectile would continue resized.

I think Bernoulli's Law is what I'd want to use to get an estimate, but folks, I simply don't remember.

I originally wanted to treat this as a simple water pipe, but can't since it's dealing with a compressible fluid and a solid being elongated.

Test firing says everything's kosher (one can get pressure signs from cases, and pressure signs usually don't appear until about 50,000psi) but that's more of a dark art than a science.

Any help here would be appreciated. I do feel like I'm leaving some vital stuff out but can't think of what it might be at the moment, so if you have any questions, please don't hesitate to ask.

Thank you,

Josh

2. Feb 3, 2012

### ardie

from what i understand from your post, the larger bullets start touching the sides of the barrel and strong forces of friction causes them to reduce in size as they move along, and then eventually exit the barrel? and this was done without changing the size of the barrel?
If this is so, then when the bullet starts swaging, the forces of friction will be so strong, that the laws of thermodynamics would not quite apply in their usual forms; the processes involved will not be adiabatic, and the particles will launch to all over the place causing chaotic motion. the intermolecular forces of the metallic barrel are very much stronger than the collective forces of the explosion.

3. Feb 3, 2012

### Wabatuckian

Hello,

The bullets grow longer when they swage. They are soft lead in the center.

Look at it this way, I guess:

A projectile of 0.318" is fired down a tube with a max diameter of 0.318". The result is, say, 35kpsi.

A projectile of 0.323" is then fired down the same tube.

The 0.323" projectile will conform to 0.318" after passing through a 1-3/8" sizer inside the barrel. Hardness of said projectile is 2.0 to 3.0 Mohs (6.0 to 11 Brinell). Easy to form.

After passing through the throat (swaging funnel), pressures should drop to 35kpsi again.

Logically, the concern should be the pressures while the projectile is being reformed.

Regards,

Josh