Case Geometry and Pressure

[Infrequent Shooter]

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.

and the air flow velocity is increased

 

[RegionRat]

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.

The studies were very external, meaning input-output cause effect was important but the details of the internal mechanics of the combustion processes were not.
Even combustion efficiency is of no concern if there is a performance benefit.

You would be shocked at how much unburned propellent goes out the end of heavy artillery, yet we don't care if that is where the ballistic trajectory performance is best.

If I were a young grad student and had some Godfather who was willing to pay for that answer, I would set that up and run it just for the kicks. Those days are long gone...

 

[gunsandgunsmithing]

The studies were very external, meaning input-output cause effect was important but the details of the internal mechanics of the combustion processes were not.
Even combustion efficiency is of no concern if there is a performance benefit.

You would be shocked at how much unburned propellent goes out the end of heavy artillery, yet we don't care if that is where the ballistic trajectory performance is best.

If I were a young grad student and had some Godfather who was willing to pay for that answer, I would set that up and run it just for the kicks. Those days are long gone...

If that's the case, it lends to explaining why "short and fat" works well. IOW, bore to case capacity ratio. I think there is a lot of truth there but not sure it's the end all, be all solution to all problems either.....And, requires more shoulder..fwiw.

 

[Infrequent Shooter]

If that's the case, it lends to explaining why "short and fat" works well. IOW, bore to case capacity ratio. I think there is a lot of truth there but not sure it's the end all, be all solution to all problems either.....And, requires more shoulder..fwi

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.

 

[gunsandgunsmithing]

Huh?

 

[3Sigma]

The studies were very external, meaning input-output cause effect was important but the details of the internal mechanics of the combustion processes were not.
Even combustion efficiency is of no concern if there is a performance benefit.

You would be shocked at how much unburned propellent goes out the end of heavy artillery, yet we don't care if that is where the ballistic trajectory performance is best.

If I were a young grad student and had some Godfather who was willing to pay for that answer, I would set that up and run it just for the kicks. Those days are long gone...

That would be a very fun graduate project.

The combustion dynamics of solid propellants are still very relevant to rocket motor design, though the empirical approach was less in favor to theoretical models in academic areas 20 years ago.

My area has been mostly liquid and gaseous combustion (diesels, spark ignition, and turbines), so all my exposure is limited to graduate coursework theoretical problems.

This is one of those topics that you don't really know how complicated it is until you really try to understand the fundamental physics and chemistry taking place. The more you learn, then less you realize you know about it all. Hell, we still can't easily model turbulent flame fronts in stratified fuel mixtures.

So, to the OP, I give Engineering Answer #1: It depends...

 

[Straightshooter1]

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?

Maybe this graph from QuickLoad gives you some idea (24" barrel)???

 

[Infrequent Shooter]

Huh?

what you are talking about. tradeoffs. short fat, so-so feeding, 7x57 made to feed in extreme circumstances. no free lunch in cartridge design.

 

[Homerange]

Case capacity to bore ratio is everything and still is despite the wider propellant choices we have today.
A case in point is the infamous 30/06 in normal dress or necked down to 6.5/06 or necked up to 35 Whelen.

Now get out your loading manuals and check what any of these 3 old cartridges can do.

The real surprise is 35 Whelen and just how efficient it is compared to the great '06 with heavier pills.
I'll leave you to do your homework and come to realise how that bigger 35 Whelen bore is far more efficient at getting that pressure and projectile down the tube.

Disclosure, I've never owned any of these 3 but instead been very interested in cartridge performance for decades.

 

[22BRGUY]

To me it is a lot like plumbing in a building.
If we say that the water pressure is a given constant 60 psi that is uniform throughout
the house or building, and if you hook up a 10 foot long garden hose to an outside faucet, that has an inside diameter of 5/8", and then turn that faucet on fully open, that water will come out at a certain velocity.
Now, with the same consistent pressure, we hook up a 3/8" id. 10 foot long garden hose,
and again turn the faucet on fully open, that water will come out at a higher velocity than with a 5/8" id. hose.
Remember, the pressure has remained constant.
What is happening is that that specific volume of water at that same pressure,
is trying to find that same "space volume" in both hoses but in the smaller diameter
hose, the water has to travel farther faster to achieve that same volume of space in the
same length of time.
The same principles are at work when a given propellent is used behind a given
weight of projectile.
As the pressure is produced by the burning rate of the powder, regardless of case
geometry, the pressure is seeking a certain volume of air space in a certain time
frame. That volume of air space is longer down the barrel of a smaller diameter
than a barrel of larger diameter.
Therefore, that smaller diameter projectile must have greater velocity. than the larger
diameter projectile but the pressure at any given time frame is the same.
JMHO FWIW.
We're talking milliseconds of time here of course.