Case Geometry and Pressure

[C_Ackley]

This question came to my attention a little bit ago, how does case geometry affect pressure?
I have an example. Lets say we have two cases, one a shorter bottleneck and another a longer straightwall, both cartridges have the same case capacity and are the same caliber.
If you put the same recipe in both cartridges, would there be an observable difference in pressure? What is the mechanism that causes that difference?

If you need a specific example, I have one too.

 

[rwj]

One would have to make a lot of assumptions to answer your question. I would cite the specific example to get meaningful responses.

 

[C_Ackley]

cite the specific example

You got it.
So 357 Maximum has a case capacity of 33-34gr of water, is a long straightwall.
357-44 B&D has a case capacity of 33gr of water, has a gently 10° bottleneck.
Looking over Hornady's data, I can't figure out why the B&D outperforms the Max by nearly 100-300fps with like weights, same barrel length, and same powders at allegedly lower pressures. (B&D is held to 42,000psi while 357 Max is allowed higher)
So it has to be case geometry, at least in my head.

 

[cmysix]

It is, why do you think states like Mich. have straight wall cartridge rules for deer hunting, not anywhere near the velocities achieved by shouldered cart

 

[RPSGuns]

Think of it like this. Picture a straight wall case as a straw. If you blow air through that straw you have a steady flow of air and is relatively easy to push that column of air. Now add a restriction to the straw by placing a finger partially over the muzzle of the straw. It takes more pressure to move that same column of air. The shoulder on a case is a restriction and it's design will affect the pressure.

 

[C_Ackley]

It takes more pressure to move that same column of air.

In this case we're seeing the opposite effect, where the bottleneck case appears to produce more velocity for lower pressure.
I tried thinking about it as a pressure washer, but that isn't exactly applicable here because they both have the same size "nozzles"(neck) and pressure(machine puts out a constant pressure), but the hoses are different size(cartridge before the shoulder).
So maybe it's just the speed at which the gases exit through the bottlenecked case mouth is faster than if it was a straightwall, thereby propelling the bullet faster?

 

[RPSGuns]

In this case we're seeing the opposite effect, where the bottleneck case appears to produce more velocity for lower pressure.
I tried thinking about it as a pressure washer, but that isn't exactly applicable here because they both have the same size "nozzles"(neck) and pressure(machine puts out a constant pressure), but the hoses are different size(cartridge before the shoulder).
So maybe it's just the speed at which the gases exit through the bottlenecked case mouth is faster than if it was a straightwall, thereby propelling the bullet faster?

I would think that your throat and lead geometry are going to affect the pressure curve at that point. Once the projectile leaves the case, the pressure curve will change depending on that particular geometry.

 

[SSL]

In this case we're seeing the opposite effect, where the bottleneck case appears to produce more velocity for lower pressure.
I tried thinking about it as a pressure washer, but that isn't exactly applicable here because they both have the same size "nozzles"(neck) and pressure(machine puts out a constant pressure), but the hoses are different size(cartridge before the shoulder).
So maybe it's just the speed at which the gases exit through the bottlenecked case mouth is faster than if it was a straightwall, thereby propelling the bullet faster?

You are on the right track. Shouldered/necked cases benefit from the Venturi Effect, which in effect causes fluids or gasses to increase in speed when passing through a constriction (shoulder/neck) while reducing the static pressure.
Your analogy of the pressure washer is actually closer than you think...again, pressure and speed of the water column can be regulated by the size of the final aperture (either by adjustable nozzles or fixed nozzle size selection) even though the hose itself stays the same.

 

[RegionRat]

If you put the same recipe in both cartridges, would there be an observable difference in pressure? What is the mechanism that causes that difference?

Yes, there are obvious differences as the above posters have pointed out.

Also keep in mind how far down the barrel the back of the bullet has gone when peak pressure is achieved and the fact that it still isn't over....

That is a sideways comment so to be blunt, the shape of the combustion chamber isn't just the chamber with the brass case, it extends down the barrel.

These are fast events with a mix from solid unburnt powder to very hot gas moving at the same time while rapidly changing and intuition and simplified laws of physics are only rough concepts when it comes to internal combustion.

You can't assume uniform pressure, density, or temperature in the chambers, and the velocity gets high enough fast enough to mean there is lots of energy in the mass flow of the expanding mix at any point down the curve.

 

[C_Ackley]

Also keep in mind how far down the barrel the back of the bullet has gone when peak pressure is achieved and the fact that it still isn't over....

Peak pressure is achieved very early in the chamber/barrel, probably shortly after the bullet has left the neck and engraved, then it tapers off over the next few ms until the bullet exits the muzzle. If you have peak pressure, or secondary pressure, late in the barrel then you have serious problems.
Did you have any thoughts as to the mechanisms that actually make the observable difference in my example?
No one disputed there was a difference, rather what causes it.

 

[dellet]

There are a lot of variables not listed here.

First is the Hornady data even comparable?
By that I mean, Hornady has two sets of data for the 357 Max. Revolver and contender. They also have a disclaimer that the contender data will show higher velocities for the same data as the revolver due to the cylinder gap. I don’t believe the the 357-44 Bain data shows bullet weights comparable to the 357 max Contender data. The 357 Bain data was Contender data. Is it possible you’re comparing that to 357 Max revolver? That would have same bullets listed.

The Bain has a very short neck, so given the same bullet seated depth as the 357 Max, it’s possible the Bain would actually have a lower load density. Bullet seated into a larger diameter cavity. Need do some math to verify.

The 357-44 will have different neck tension, less bearing surface in the neck. Lower start pressures, changing the pressure curve. The restrictive bottle neck, may or may not compensate for that. Loads with a higher start and peak, run out of gas sooner. It’s not that they start slowing down, but after peak they accelerate slower than a load with less peak. Pressure curves would really need to be compared along with percentage of powder burned.

There’s a lot more going on than a 10* shoulder, same bullet, same powder charge in this comparison.

 

[gunsandgunsmithing]

Lots of factors beyond powder volume and a shoulder that can account for the mentioned velocity differences. I don't think the shoulder has much, if anything to do with it. It might a small amount but we've all seen some bbls be faster than others, we've seen bbls speed up, conditions tested in, freebore length, leade angle, powder lot variations...just a lot of factors beyond a shoulder, imho. Some of these factors can be worth that much alone, and certainly so, cumulatively.

 

[C_Ackley]

We've seen bbls speed up, conditions tested in, freebore length, leade angle, powder lot variations

I agree, and my example was really only to provoke thought back to the hypothetical.
Suppose someone controlled for these factors. Two barrels(or more depending on the number of shouldee angles) of the same make mated to a test rig, different chambers, two cartridges(or many more of varying shoulder angles to map out the effecr more clearly) one straightwall and one bottlenecked of equal case capacity and caliber, same length throat from the bullet.
I'm just interested to know how the geometry of a case affects the manner in which pressure and velocity are generated relative to a straightwall cartridge.
The venturi effect mentioned above makes sense, but there are too many confounding factors in my real world example to say that accounts for even a portion of the variance.
So I'm kind of fishing to see if anyone has done this kind of research and if someone has a link to it. The experiment would be prohibitively expensive, that much is apparent.

 

[gunsandgunsmithing]

I agree, and my example was really only to provoke thought back to the hypothetical.
Suppose someone controlled for these factors. Two barrels(or more depending on the number of shouldee angles) of the same make mated to a test rig, different chambers, two cartridges(or many more of varying shoulder angles to map out the effecr more clearly) one straightwall and one bottlenecked of equal case capacity and caliber, same length throat from the bullet.
I'm just interested to know how the geometry of a case affects the manner in which pressure and velocity are generated relative to a straightwall cartridge.
The venturi effect mentioned above makes sense, but there are too many confounding factors in my real world example to say that accounts for even a portion of the variance.
So I'm kind of fishing to see if anyone has done this kind of research and if someone has a link to it. The experiment would be prohibitively expensive, that much is apparent.

I've done just a little but there were variables. I'm of the belief that a shoulder wastes energy more than adds to it, directly or indirectly..fwiw. I did some pressure trace work with a 30br vs a 30 Major(Grendel. The grendel holds a couple of grains less powder and the case is ppc diameter. The pressure trace yielded some interesting info relative to case efficiency and the Pressure Trace designer told me that a 30 Major "may be the most efficient case he's ever seen data on." fwiw...Area under the pressure curve was what he was looking at specifically. Same angle as a br but less shoulder due to the case being skinnier. Granted, a little less volume plays into it too. Can't get quite to br speeds but close...so I don't think there is much value there..but some.

 

[gunsandgunsmithing]

edited

 

[gunsandgunsmithing]

Here's a link to an overbore comparison chart from this site. The least overbore are most efficient. The top several on the chart are literally a who's who of the most accurate cartridges in the world...except one...a 458 win mag(no shoulder) Hmmm.

Judging by your screen name, I'm guessing you're hoping for something different but I'm only telling you what very little I know on this subject, for sure and not so sure. Lol!

You are being redirected...

 

[C_Ackley]

Judging by your screen name, I'm guessing you're hoping for something different but I'm only telling you what very little I know on this subject, for sure and not so sure. Lol!

You are being redirected...

My name ultimately doesn't mean anything, I didn't find out about P. O. Ackley until I had started loading. I'm a know-little asking the giants I stand on what they know to see if I can get a response.

 

[gunsandgunsmithing]

My name ultimately doesn't mean anything, I didn't find out about P. O. Ackley until I had started loading. I'm a know-little asking the giants I stand on what they know to see if I can get a response.

I know dang near nuttin but I look at long necks the same way. Lots of people claim better bbl life from a neck that's 1/16th of an inch longer when 65000psi and several thousand degrees of heat are what eats up bbls. I'm just a redneck ky dummy, so you're forewarned. Degrees is how hot it is, right?

 

[M-61]

For me this is one of the best posts in a very long time.
Whole world of what or how little I know.
So here’s my question:
If the pressure of a cartridge produces 50,000 psi
that is measured in the chamber? What is the pressure the moment before the bullet exits the muzzle? The volume is constantly growing as the bullet goes down the barrel?

 

[gunsandgunsmithing]

For me this is one of the best posts in a very long time.
Whole world of what or how little I know.
So here’s my question:
If the pressure of a cartridge produces 50,000 psi
that is measured in the chamber? What is the pressure the moment before the bullet exits the muzzle? The volume is constantly growing as the bullet goes down the barrel?

Depends on other factors. Burn rate being a biggie. Burn rate is something you can SEE a big difference it. No, can't see it under pressure like this but there's a huge difference in how different powders burn, just in a pile even. Slower burns longer, faster burns..faster. The difference in open air is significant. fwiw.. If it's anything close to that under pressure, burn rate is a massive variable. I'd say the difference can range from roughly 2000psi to 15000psi at the muzzle