Help me understand barrel length / Velocity

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oldsparky

oldsparky

Member
Feb 15, 2018
1,185
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arizona, United States
I have reloaded and played with powder burners for 40 years and have a somewhat understanding of them. I have had a PCP for 7 years but just to practice and didn’t really care about how it worked. Last year I bought another rifle and would like a pistol but haven’t found exactly what I want. What I am having a hard time understanding is velocity / barrel length. On a smokeless powder barrel the length of the barrel must be long enough for the powder to completely burn to get the energy from the powder. Back in the 80’sshooting Contender pistols with rifle cartridges using factory ammunition you might get a flame several feet out of the barrel because the powder wasn’t finished burning. A solution would be to load a powder that had a faster relative quickness making the powder burn inside the barrel without wasting energy transfer. What I don’t understand from reading about velocity loss from a airgun is why does a shorter barrel within reason lose velocity as you cut off inches from the barrel. It seems there would be no increase of energy like the powder still burning but maximum pressure would be obtained in a relative short distance. After maximum energy is transferred to the pellet it would seem barrel resistance would slow the pellet. Or is it because we are trying to conserve air that the valve opening sizes are limited so the blast of air isn’t what It is in my mind. I hope I put to paper what is in my mind and wasn’t just confusing.



Thanks, Jim
 
I believe it was Tony of TalonTunes who explained the dynamic to me this way:

"The longer the barrel of a gun the more time the compressed air imparts its energy to the projectile", thereby providing more velocity/power to the projectile.

From AOA site promoting the 700mm barrel:

"All air rifles operate on the same principle: pressurized gas (usually air, but sometimes CO2) is introduced into the barrel behind a projectile, and the pressurized gas drives the pellet or BB down the barrel and out the muzzle."

I hope that helps.
 
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The port size, available air(plenum or reservoir size), valve dwell time and barrel length all affect velocity. A short dwell time and/or port size won't usually realize higher velocities (to any great degree) with a longer barrel. All this is part of the tune or design process. It is similar to loading a cartridge with different rate powders to optimize velocity in a particular barrel. One rifle I am working on uses quite a bit of air for each shot and manifests that with LOUDNESS and an aftershot push. I need less dwell time and a larger port plus lower reg pressure to keep the velocity where I want it but use less air, it seems.

Hope this helps some,

Bob
 
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Thanks Bob, that makes sense. Our stored energy is a constant. It doesn’t increase. So all we can control is the quickness and volume of our stored energy transfer. The reason I get pondering this is because I would like a pistol that I can’t find. I want something light and not the 18 inch barrels I see. Also I am glad you mentioned noise. What I want in my mind would be loud. So that defeats the benefit of an air pistol for my wants. 



Thanks, Jim
 
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Buy a Leshiy
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Always good advice ?
 
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airmax mk 2 .22 pistol aka pp700sa. great pistol barrel is only around 9 inches lots of tuning and middling potential but great shooter right out the box under 200 bucks gets around 30 shots. I love mine. I put a short 4
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inch ldc on it and it works suprisingly well for being so small
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I second this choice. I have one in .177 with a red dot on it. I use it for pistol field target. If I miss it's my fault.
 
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