Idea for better pellet speeds

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If you're interested, Bob has written a lot on this subject over on GTA. One such example:
Springers are a "different breed" than pneumatics (PCP, Pumper or CO2)…. With a Springer, the mass of the air is quite small compared to that of the pellet.... A swept volume of 50 cc contains only about 1 grain of air.... The energy comes from the Adiabatic compression of that air (ie when you compress it suddenly, it heats up, and that heat increases the pressure even more)…. The peak pressure reached has a lot to do with the FPE.... Generally, Springer have a range of pellet weights that produce the highest FPE.... but pellet fit enters into it as well.... If the pellet is too light or too heavy.... or too loose in the bore or too tight.... the peak pressure is less, and so is the FPE....

Pneumatics work by expanding the air or CO2.... The pressure goes from high to low, and hence the gas cools, which actually reduces the pressure and energy, the reverse of a Springer.... The more powerful the gun, the more gas is used to produce the FPE.... Unfortunately, that expanding gas has to accelerate not only the pellet, but also itself.... How much energy is lost doing that depends on the mass of the gas released, and how it expands.... A typical PCP uses more than 10cc of air (at atmospheric pressure) for every FPE it produces.... Big Bores might use twice that much.... A 20 FPE gun would discharge at least 4 grains of air behind the pellet on every shot.... A 200 FPE Big Bore might discharge as much as 80 grains of air per shot....

Obviously, accelerating that air takes energy, and that energy is not available to accelerate the pellet or slug.... That 20 FPE PCP, shooting an 8 gr. pellet would be wasting a higher percentage of power on the air than the same PCP accelerating a 16 gr. pellet.... Therefore, the 16 gr. pellet will end up with more of the total energy available, and the gun will be more efficient shooting the heavier pellet.... The best example of this was my 6mm PCP.... When tuned for maximum power at 4200 psi, I got 175 FPE with a 73 gr. lead slug.... Without changing anything, the same gun fired a 1.8 gr. plastic airsoft BB at 2092 fps, but that was only 17.5 FPE.... just 1/10th of the energy.... The lost energy went to accelerating the air, which in this case weighed more than the lead bullet....

The entire bore volume will not achieve anything close to full operating pressure.
In his examples, he relates the mass of air to the energy but if we assume the 20fpe example were based on a 16gr pellet, the 4gr of air used to accelerate it represents 25% of the pellet's weight.

A couple of relevant threads (the first one is where I copied the quote above).

Why do heavier pellets tend to shoot at higher energy than lighter pellets? - Airguns & Guns Forum

Why do heavier pellets tend to shoot at higher energy than lighter pellets?, Air guns, Airgun Forum
www.gatewaytoairguns.org

last of the internal ballistics formulae - Airguns & Guns Forum

last of the internal ballistics formulae, Air guns, Airgun Forum
www.gatewaytoairguns.org
 
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JDGriz said:
I actually know the efficiency of my own .22 air guns: it is about 0.40 to 0.45.
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What units are you using? Airgun efficiency is typically expressed either as fpe/ci or bar*cc/fpe

For small bore airguns, a fair efficiency figure might be something like 1.0fpe/ci or 16.4bar*cc/fpe
A good efficiency figure might be something like 1.5fpe/ci or 11bar*cc/fpe.
 
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JDGriz said:
It is reasonable to assume that the limiting factor for pellet speeds in air guns is the speed of sound in the barrel. If you just let compressed air expand behind the pellet, then the pellet would only accelerate until its speed matches the speed of the expanding air, and the speed of the expanding air can't be faster than the speed of sound.
But, if you impart velocity onto the expanding air itself, then this might be able to compensate for this effect.
A bit like what they are doing in this video:

So, creating a nozzle behind the pellet might actually make it go faster, even if it would be a bit of a choke point. This could actually be quite easy to do on most air guns, by incorporating a choke into the probe (to create a nozzle with the barrel walls), assuming much less air resistance all the way through the port, valve and to the air tank (i.e. higher cross-sectional area).

like this:

View attachment 406778

(the second choke version is intended to be like the ports on some springers, which have stepped changes in diameter to induce eddies and reduce air resistance)
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Not this again. Air travels faster than the speed of sound. Air and the sos have absolutely nothing to do with one another.
 
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elh0102 said:
I know virtually nothing of the math behind all this, and I care even less. But years ago, maybe 10+, I remember reading the results of some PCP velocity tests, and those guys got well into supersonic territory, 1,300+ fps. So, are we sure that Mach 1 is the limit?
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Simple answer:
Mach1 (~1116fps) is not a hard limit. You can launch projectiles beyond Mach1 using air ...and... IF...If...if...(if) they remain stable in flight, you will have to account for more wind drift, a lot more than at subsonic speeds. That about sums it up for the simple answer. 😅
 
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JDGriz said:
It is reasonable to assume that the limiting factor for pellet speeds in air guns is the speed of sound in the barrel. If you just let compressed air expand behind the pellet, then the pellet would only accelerate until its speed matches the speed of the expanding air, and the speed of the expanding air can't be faster than the speed of sound.
…
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The maximum mass flow rate of air is limited by the sonic condition, i.e. Mach 1. But once that column of air is flowing in a tube, the front can undergo additional velocity increase via expansion, while the mass flow rate stays constant. The velocity limit of the front can be a little over twice the speed of sound of the source air under certain conditions. This is for flow/expansion in a tube(barrel).

Given a low mass projectile, there are many airguns that can exceed the speed of sound of the source air.
 
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Scotchmo said:
The maximum mass flow rate of air is limited by the sonic condition, i.e. Mach 1. But once that column of air is flowing in a tube, the front can undergo additional velocity increase via expansion, while the mass flow rate stays constant. The velocity limit of the front can be a little over twice the speed of sound of the source air under certain conditions. This is for flow/expansion in a tube(barrel).

Given a low mass projectile, there are many airguns that can exceed the speed of sound of the source air.
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Sure, the question was, if you could get 40% more fpe by just making a better probe (and port and valve), would it be worth the increased technical complexity? I had as much as 60% of energy input unaccounted for, in my general understanding, but the weight of HPA behind the projectile can take care of all that, and I had missed it.
40% would have been worth it, but less than 5% isn't (if it is that at all, at normal speeds).
 
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1700202662991.jpeg
 
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JDGriz said:
Sure, the question was, if you could get 40% more fpe by just making a better probe (and port and valve), would it be worth the increased technical complexity? I had as much as 60% of energy input unaccounted for, in my general understanding, but the weight of HPA behind the projectile can take care of all that, and I had missed it.
40% would have been worth it, but less than 5% isn't (if it is that at all, at normal speeds).
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My response was strictly dealing with the incorrect premise to your question. Knowing that, you might rethink the question.
 
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Scotchmo said:
My response was strictly dealing with the incorrect premise to your question. Knowing that, you might rethink the question.
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If I understand this correctly (now), a nozzle increases flow velocity, at the expense of pressure and additional expansion directly behind the projectile, mass flow being constant. So you are actually incurring more losses in the form of wasting energy to accelerate air, to make sure that pressure is constant behind a projectile, which would have been moving closer to the flow speed had there not been a nozzle. But it's a lower pressure behind the projectile, than it would have been without the nozzle. So basically a nozzle is just a gearbox, which trades torque(force,pressure) for speed, except that unlike gearboxes you lose energy to accelerate things you don't need to accelerate (HPA).
This would only make sense if you are chasing projectile speed, irrespective of projectile energy, but doesn't make too much sense if you want to optimize for projectile energy or efficiency in the classical sense. And actual mach numbers and speed(s) of sound are mostly irrelevant to the discussion.
 
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