I keep reading about increasing barrel length will increase velocity to a point and then have no effect. Is there a formula that gives increase per inch of barrel? I'm sure that caliber has to also be in the mix. Does a fast starting speed have more effect because there is more pressure or volume of air?
Tuning Barrel length vs velocity
- Thread starter marquette
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Im not sure on a formula. But the bigger the plenum you’ll see some good speed with longer barrels. On a tuned uragan I was still gaining speed at 35 inches.
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I generally go by between 12-15 fps per inch in general based on swapping barrels in the past with no other modifications.
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There certainly could be a formula, but I think it would be impractical to have a universal one applicable to all barrels. One of the biggest unknowns would be the amount of friction for the particular pellet/barrel combination, and unless this was known, a universal formula would probably be subject to a wide margin of error.
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I do believe that someone like Motorhead has posted up a formula dealing with that question.
Maybe it was Ackuric, Or Akuric, or something similar as a screen name?
I do believe that there is a point of diminishing returns that goes with the barrel length issue that has been spoken on as well.
The place to inquire would be over on the GTA, and pose your question in the Bob and Lloyds Workshop forum. You will probably learn more than you ever realized you never wanted to know.
Maybe it was Ackuric, Or Akuric, or something similar as a screen name?
I do believe that there is a point of diminishing returns that goes with the barrel length issue that has been spoken on as well.
The place to inquire would be over on the GTA, and pose your question in the Bob and Lloyds Workshop forum. You will probably learn more than you ever realized you never wanted to know.
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I’ll keep using search now that I know that there has been more posted than I have found. I’ll also post on GTA to see what I learn there. I guessed 10-20 FPS would be the average
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Doing a simple search @ackuric has done a lot of research on this topic. Here is one example:
https://www.airgunnation.com/topic/official-release-simple-pcp-calculator-mobile/
And at least where I first ran across his thinking:
https://www.airgunnation.com/topic/measuring-internal-ballistics/
I found those pretty quickly using this search:
https://www.google.com/search?q=site%3Aairgunnation.com+barrel+length+formula
https://www.airgunnation.com/topic/official-release-simple-pcp-calculator-mobile/
And at least where I first ran across his thinking:
https://www.airgunnation.com/topic/measuring-internal-ballistics/
I found those pretty quickly using this search:
https://www.google.com/search?q=site%3Aairgunnation.com+barrel+length+formula
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I read the first 2 links and they were pretty complicated stuff. At the end of the 3rd link there was a post showing the difference between a 50 and 45 cm barrel The post showed it took 10 bar more pressure in the 45cm barrel to reach the same velocity as the 50 cm barrel. But they don’t say the velocity of 45 cm barrel to start with. I’ll bet though it follows in line with Odoyles observations.
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With springers and gas rams there is a sweet spot where an maximum velocity occurs because of the balance between drag and available power. Sometimes shortening the barrel on a springer will give you more velocity.
On my TalonSS with a 12" barrel in .25 and .20 I observed there was a maximum achievable velocity and no matter how much more air I dumped I had basically hit a wall. The 24" .22 barrel shoots faster than either of the 12" barrels and uses less pressure and less air to do it.
On my TalonSS with a 12" barrel in .25 and .20 I observed there was a maximum achievable velocity and no matter how much more air I dumped I had basically hit a wall. The 24" .22 barrel shoots faster than either of the 12" barrels and uses less pressure and less air to do it.
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I agree that every barrel length/gun would have a sweet spot. Otherwise we would see 60 inch barrels. I’m sure when we keep trying different weight pellets, pressures, and hammer stroke we are in effect just looking for the barrels sweet spot.
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Here are two very interesting articles:
This one doesn't go into great depth but makes helped me understand.
https://www.thefreelibrary.com/Airgun+barrel+length.-a0310520508
And this one does not address barrel length specifically, but helped me understand what is going on in a PCP:
https://hardairmagazine.com/ham-columns/pcp-airgun-internal-ballistics/
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This one doesn't go into great depth but makes helped me understand.
https://www.thefreelibrary.com/Airgun+barrel+length.-a0310520508
And this one does not address barrel length specifically, but helped me understand what is going on in a PCP:
https://hardairmagazine.com/ham-columns/pcp-airgun-internal-ballistics/
- G
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It's simple. Velocity scales with pressure. A longer barrel means more time for pressurized air to push the projectile, but it also requires more pressurized air volume for a given velocity. More pressurized air volume means higher pressure or a bigger plenum / higher VDT.
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Wouldn’t there be a fourth factor in the equation for the graphs in the HardAirMagazine article? Caliber? A larger bore would contain more air volume per inch. Is there a correlation between pellet diameter and surface area for the air to apply its push? Or is just simply the air volume behind the projectile? Need more coffee.
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