Velocity vs Chamber Pressure

[NapalmCheese]

I feel Barsness put it pretty succintly. More velocity ALWAYS means more pressure, everything else being equal.

When you don't keep everything else equal: longer barrels generally just mean there's more time for the bullet to be pushed by expanding gases.

Think of it this way, what if your friend's crappy Geo Metro won't start again and they ask you to push the car. From a dead start, if you push as hard as you can for 1 second how fast will the car be going? What if you're allowed to push as hard as you can for 5 seconds? What if you're allowed to push as hard as you can for 60 seconds?

In the first instance you can't push the car long enough to achieve maximum velocity. In the second instance maybe you DO have enough time to achieve maximum velocity. In the third instance you're unlikely to maintain maximum output for 60 seconds straight and at the end of it you'll likely be pushing the car at a sub maximal velocity (having peaked earlier).

[bubbapug1]

Very good analogy! I would add all powders are basically the same, except the geometrical shapes are different to alter burn rate profiles. You also might have burn retardants, etc.

At first ignition peak chamber pressure is reached, than its a game of gas production versus bullet speed to try to goose the bullet as long and hard as you can before it exits the muzzle.

In a pistol you don't have a lot of time so pistol powders just give it up quickly, reach peak chamber pressure, and give up. Other powders, like Hodgdon H4831SC reach peak pressure, but also keep providing pressure via gas production almost down the entire length of the barrel.

Remember, as the bullet goes down the barrel you have the ideal gas formula working against you, and if gas is still not being produced, every distance in chamber length results in a decrease by half of the beginning pressure. So if the peak pressure was 50,000 psig in a 2" chamber, at bullet travel of 4" the pressure will fall to 25,000 psig. That is a simplification as there are other forces at work such as engraving pressure to start the bullet into the rifling, the resistance to the bullet by the rifling (friction), etc. When the gas pressure becomes less than the friction of the rifling the bullet will begin to slow down, so you can't just add barrel length to go faster, you need to add gasses too, and in ever increasing volume....

I feel Barsness put it pretty succintly. More velocity ALWAYS means more pressure, everything else being equal.

When you don't keep everything else equal: longer barrels generally just mean there's more time for the bullet to be pushed by expanding gases.

Think of it this way, what if your friend's crappy Geo Metro won't start again and they ask you to push the car. From a dead start, if you push as hard as you can for 1 second how fast will the car be going? What if you're allowed to push as hard as you can for 5 seconds? What if you're allowed to push as hard as you can for 60 seconds?

In the first instance you can't push the car long enough to achieve maximum velocity. In the second instance maybe you DO have enough time to achieve maximum velocity. In the third instance you're unlikely to maintain maximum output for 60 seconds straight and at the end of it you'll likely be pushing the car at a sub maximal velocity (having peaked earlier).

[LynnJr]

Velocity is pressure over time.
In a 2 inch snub nosed pistol your barrel is extremely short so you need as much pressure as you can safely put behind the bullet in those 2 inches.

With a target rifle your barrel will be 15 times longer so you can use a very slow powder to get a longer push against the bullet and thus higher velocity.

[JagerDog]

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.

Unique is often a good choice for pistol caliber carbines. Seems CFE pistol is a bit slower so should be good too. At some point you can't stuff enough of the slower stuff to make good use of the burn properties.

[pacrat]

[1].... Is there always a direct relationship between the two?

[2].... In data for a specific load it shows the velocity and pressure for starting and max charge in a specific barrel length. [3].... How does that relate to a different length barrel? [4].... Let's say a shorter barrel, 3" will have less velocity and less pressure I assume, and a longer barrel more.

[5].... 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? [6].... 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?

[7].... Likely overthinking things.

Velocity is pressure over time.
In a 2 inch snub nosed pistol your barrel is extremely short so you need as much pressure as you can safely put behind the bullet in those 2 inches.

With a target rifle your barrel will be 15 times longer so you can use a very slow powder to get a longer push against the bullet and thus higher velocity.

[1].... YES, but how that relationship affects performance is governed by other variables also. Which are as relevant as they are variable.

[2].... Note that your hypothetical "specific load data" is only applicable to that "barrel length".

[3].... That depends on the sum of all the variables.

[4].... Don't ever assume. There are circumstances that can give differing results. Ex... the load is optimal for 3" like 2.5 gr Bullseye. PEAK pressure will be same but velocity can be lower at 3" or 14".

[5].... No. Again Peak pressure will be same. See what Lynn said about TIME. More barrel means more time. More time REQUIRES more pressure, it doesn't CREATE more pressure.

[6].... It happens. Think of it this way. A given amount of powder generates X amount of [PLASMA] at ignition. PLASMA is the key. That X amount of powder generates the energy in the form of SUPER HEATED EXPANDING GAS aka PLASMA. That PLASMA is finite. If the load doesn't create enough plasma to PUSH the bullet the full length of the barrel. The bullet slows before reaching the muzzle.

With the very limited case capacity of a 9x19. Likely the Plasma has pooped out and quit expanding and creating pressure at [WAG] of 8-10". Just adding barrel only increases velocity to the, Point of Diminishing Return. Without LONGER push time, LONGER barrel becomes a hindrance to velocity.

That is why BURN RATES are so very important. Big cases with big powder loads HAVE to burn slower to generate PLASMA over a longer period of time. Without exceeding MAX PEAK pressure. If ALL THE ENERGY in a big load was released in the same millisecond. The resulting plasma generated would KaBoom the gun.

ETA.....That "bubbapug1" also did a very good job of describing the variable load relationships and requirements. I brainfarted and didn't read his post before I started typing.

[five.five-six]

The further down the barrel the pill travels, the more displacement there is behind the pill and the more gasses required to maintain the same pressure

[J-cat]

Look it: increasing barrel length increases velocity. It does not increase pressure. Increasing pressure in the same length barrel will increase velocity. But a longer barrel will never increase pressure unless it has a tighter chamber. A 9mm 16” barrel will pick up 200 FPS over a pistol barrel with several powders. I use Power Pistol for this. Very accurate.

[DueceMcGurk]

OP, think pressure curve not max pressure. Then think area under the curve. Add in about 6 variables and bingo. You are now an "Inga Neer".

[five.five-six]

OP, think pressure curve not max pressure. Then think area under the curve. Add in about 6 variables and bingo. You are now an "Inga Neer".

OK, I did all that, do I get to drive the train now?

[JagerDog]

OK, I did all that, do I get to drive the train now?

Doesn't this belong on the AOC tune-up thread?