Velocity vs Chamber Pressure

[Donny1]

Is there always a direct relationship between the two?

In data for a specific load it shows the velocity and pressure for starting and max charge in a specific barrel length. How does that relate to a different length barrel? Let's say a shorter barrel, 3" will have less velocity and less pressure I assume, and a longer barrel more.

What happens with the same load in a PCC barrel at 17" in 9mm where the velocity is 100-150 fps more, is that more chamber pressure? And a scenario that I've read about where a pistol cartridge in a long barrel will lose some velocity before it leaves the barrel. For example, if at 14" it's 1350 but at the muzzle it's only 1200, how is the pressure calculated?

Likely overthinking things.

[hermosabeach]

Google

Ballistics by the inch

The shoot and chrono loads and then chop the barrel and repeat

[Donny1]

Google

Ballistics by the inch

The shoot and chrono loads and then chop the barrel and repeat

I'm not chopping anything!

[hermosabeach]

http://www.ballisticsbytheinch.com/

You don’t have to- they have already run tests that might answer some of your questions


9mm starting at 18” and chopped down to 2”
Different loads and velocity

http://www.ballisticsbytheinch.com/9luger.html

[hermosabeach]

The pressure is chamber pressure.

Rifles normally have longer barrels. They use a slower burning powder so the pressure can build for a longer period.

Some pistol powders, like bullseye, ignite and burn much faster so you can reach peak pressure in a few inches.


I’m not sure of your real curiosity as far as pressure.

If you have a PCC you can look towards the slowest powders to possibly increase velocity. You might look at the powders used for heavy .44 mag loads- like 2400- and see if you can find a saami approved load for 2400 in 9mm

As all cartridges are based upon an industry standard -saami- all 9mm loads will be similar.


The exception are loads designed for things like the Thompson contender. You can find .357 mags that are only for the contender. They might KB a normal revolver. The contender is a beefy single shot and can handle more Kpsi



In any semi auto, once the breech opens, your pressure falls off.

So to max velocity from a semi auto, at what point does the breech open?

Is it blow back? Gas port?

If pressure is high when a semi auto tried to extract a round, in a rifle, case head separation would be a concern as would be breaking an extractor.

[Donny1]

Thanks!

[Donny1]

Interesting info for sure. Looks like the 17" barrel is right around the length some of the rounds are decelerating, good to know. Not really an answer as to pressures but good enough for a start.

[JagerDog]

[Donny1]

So my takeaway from that is that something equal to the specs they use in a specific load's data will be safe in any chamber/barrel length. I was just curious if it was safe pushing these pistol loads close 1400 fps in the PCC or if I should slow them down.

[Sandspider500]

Aa#9 may work well for you in the pcc, loads do exist. It may not give top velocities in your pistol due to its burn speed though. As was said before your load books are giving you peak chamber pressure, a 9mm pistol chamber is just like a 9mm pcc chamber, a 30k pistol load is going to be a 30k pcc load. As the bullet leaves the case the combustion chamber is increasing in size, so pressure is dropping.

A load with bullseye will hit its peak fast and quickly drop off, it's spent all it's energy where as a load with a slower burning powder such as aa9 will hit its peak and drop down slower still pushing the bullet. Out of a short barrel this won't matter much as it will be wasted energy.

As an example 3 different 45 colt loads using the same bullet, a 300gr gas checked wfn out of a short 4 5/8" barrel all 3 have around the same chamber pressure.

1. 13gr hs-6 for 1056fps avg
2. 18gr 2400 for 1034fps avg
3. 21.6gr win 296 1052fps avg

Pretty much the same velocity results from a short barrel, but these same loads chrono on the same day out of a 20" barrel

1. 13gr hs-6 1355fps avg
2. 18gr 2400 1378fps avg
3. 21.6gr win 296 1510fps avg

The 296 was still pushing in the longer barrel.

[JagerDog]

So my takeaway from that is that something equal to the specs they use in a specific load's data will be safe in any chamber/barrel length. I was just curious if it was safe pushing these pistol loads close 1400 fps in the PCC or if I should slow them down.

Safe yes (though always start reduced).

For most intents and purposes, a 9mm chamber is a 9mm chamber. Now when you get into high powered rifles with gobs of powder the slight differences (can) get magnified.

Might stay away from cast or plated bullets at those velocities.

[Donny1]

Safe yes (though always start reduced).

Might stay away from cast or plated bullets at those velocities.

I went out today with a lower charge and got 1200fps and a lot less snappy, just about perfect.

FYI
Xtreme 124gn CPFP
4.8 gn CFE Pistol
1.070

[hermosabeach]

You have another trade off. Accuracy vs speed

The max load normally does not provide the best accuracy.


With the long barrel, I’d wonder what speeds you could load the 147/150 projectiles.


You might get more energy from a heavy bullet as the longer PCC might give you a better gain over a 3.5” pistol reloading g with a slow burning powder.

[Donny1]

I'm going to find out, I have an order in that has some 147's in it.

From a chart I saw the CFE Pistol is not as fast as some other pistol powders, accuracy has been very good.

[Sandspider500]

Think slower