Why is the efficency of pcp air rifles so low compared to firearms?

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Xplorer

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Dec 3, 2020
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Ahvenanmaa, Finland
About 30 % of the chemical energy from gunpowder is transfered to kinetic energy of the bullet.

So the percantage of the pressure energy transfered to the kinetic energy of the bullet is even higher.

A top quality pcp transfers roughly 21-22 % of the pressure energy to kinnetic energy of the slug or pellet.

My intuition was that it would be the other way around. That an air rifle would have better efficency not worse since allot less energy is lost to heat.

Where do the rest of the energy go?

Less recoil. Less heat. One would think the losses would be smaller.

But they aren't.

Why?
 
In powder burners the explosion and force pushing the projectile happens in the cartridge. The bullet is sealed very tight and gets the best chance possible to catch the gas behind it and get it moving forward. With air rifles we are sealing to the barrel, and have ALOT of wasted air going past the projectile when its at the most critical time (starting from a dead stop)
 
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A larger airplane wing is more efficient in creating lift than a small one. Why? Because the scale of the wing changed but the size of the air molecules did not and the farther you get away from the air molecule size the more efficient the wing is in creating lift.

I think it is a matter of scale. You are scaling the pressure. What about bullet weight, friction in the barrel, the afore mentioned air molecules in the barrel, inertia, gravity and even time itself? I would think they, and many others, would all play a part.
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Your info on the size of an airplane wing is 180 degrees out! The larger a wing is, the plane is usually much slower as a result. Some of this is because the larger wing is necessary to provide the lift for a bigger, heavier plane to boot. I flew and worked in the aircraft industry for over 35 years, and the smaller the wing, the faster the plane. You can go back to military models, the Starfighter was by far the fastest plane of it's era, and had a knife edge stubby little wing that was so sharp it could actually cut you. I worked for Mitsubishi Aircraft Intl. for over 5 years, and our turboprop wings were small, short, and thinner than the competition...and we were the fastest turboprop on the market. Huge flaps were necessary to provide the lift necessary for takeoff and landing at lower speeds. The small wing did not provide good handling characteristics, but it was fast!
 
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Here's a weird question maybe. How much speed does a bullet pickup JUST from the pop off the end of the cartridge? If you had a .22 lr with NO BARREL, just the cartridge, how fast would that bullet be going when it fires?


I was thinking - all of the push in an airgun happens as the pellet travels down the barrel. But a firearm... the pellet is accelerated initially by the pressure of the gas contained in the cartridge before it starts being accelerated by the gas behind it in the barrel... not sure if this is a factor or not but it's like a two stage boost in a firearm. A little mini barrel (cartridge) stage first...
 
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Here's a weird question maybe. How much speed does a bullet pickup JUST from the pop off the end of the cartridge? If you had a .22 lr with NO BARREL, just the cartridge, how fast would that bullet be going when it fires?


I was thinking - all of the push in an airgun happens as the pellet travels down the barrel. But a firearm... the pellet is accelerated initially by the pressure of the gas contained in the cartridge before it starts being accelerated by the gas behind it in the barrel... not sure if this is a factor or not but it's like a two stage boost in a firearm. A little mini barrel (cartridge) stage first...
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Use a Chrony to measure the fps at 30 mts, shooting the same ammo ( a 22 LR) from a handgun (pistol) and from a rifle.

There you will have your answer.
 
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There's several good answers on here already. Another thing to consider is port location, shape and size in relation to the valving, barrel length, and projectile size and weight. Making high pressure air flow the most efficient from the initial opening of the valve, not having it turning sharp corners abruptly as it expands, and having as much barrel length as needed to sufficiently propel the size and weight of a projectile all matters in the ending efficiency numbers. This is why some seemingly similar airgun setups see better PFE numbers pushing the same sized projectile than others do, even within the same models of airgun.
 
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Heat is also energy. As the air expands down the barrel it cools and looses energy. Obviously its more complex than that.
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This, plus the heat loss as the air travels out of the regulator, through the transfer port, and down the barrel. Flowing around the ‘turns’ is inefficient, causes heat, and therefore losses in an airgun. Therefore there are a bunch of losses to even get the air to begin pushing the the pellet down the bore.

As previously stated, a longer barrel allows for more time to transfer the expanding air energy to the pellet-but the same is true for a rimfire as well (to a point, in both cases).


The rimfire cartridge, in contrast, is a straight shot down the barrel-making it a much more efficient path for the gasses to expand down.

Sean
 
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