What causes recoil on a pcp ?

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tor47

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May 18, 2018
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Other, Norway
I sometimes wonder what is the reason for the relative high recoil on my impact shooting in .25 cal. It seems to have more recoil than a .22 rimfire, even the energy is lower. I believe it is not the weight of the projectile, but rather the weight of the compressed air moving inside the gun causing it?

When dry firing, the gun has practically no recoil. My guess is then, the air inside the barrel never get compressed by the resistance of the pellet, and simply "escapes" freely not able to build up a pocket of heavy compressed air. When firing a pellet, the air inside the barrel still has high pressure and weight, and when moving forward in the barrel it is constantly pressing against the pellet probe, adding more weight as the pellet moves down the barrel, and making the gun to react in the opposite direction.

So are my assumption and understanding right, or is there other reasons? 
 
Newton's third law 👊

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Matt
 
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Newton's third law 👊

1590272483_12774989995ec9a1e3399746.31346520.gif


Matt
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Yes, but instead of "force on ball", it would be "force on ball, and air". The air in the barrel which I guess is heavier than the pellet, also has to be propelled forward? A powder burner use other gas than air, which is probably lighter, and thereby less recoil.
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Air rarely surpasses the weight of the projectile in an airgun, and when it does..you'll likely be going supersonic o_O because the projectile will likely match the speed of the average air molecules at whatever average barrel pressure.



Theres also slight 'hammer' recoil, heavier hammers create more, but its nothing compared to the recoil from air and projectile leaving the barrel.




 
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Recoil? Take the same gun shooting the same bullet at the same velocity with different gun powders and you can have a difference in felt recoil. Powder burn rate. Relative quickness changes the dwell time of the bullet in the barrel, the flash out the barrel and the noise. Each one of those can change the felt or perceived recoil. I don't know much about air guns. Can air volume, and air pressure change getting the same velocity causing a variable? Interesting question. I am interested in the answers ahead.
 
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Recoil? Take the same gun shooting the same bullet at the same velocity with different gun powders and you can have a difference in felt recoil. Powder burn rate. Relative quickness changes the dwell time of the bullet in the barrel, the flash out the barrel and the noise. Each one of those can change the felt or perceived recoil. I don't know much about air guns. Can air volume, and air pressure change getting the same velocity causing a variable? Interesting question. I am interested in the answers ahead.
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I am not a gun expert myself. But without having a direct comparison, I would say a .25 pcp will probably have more recoil than a .22 rimfire with standard ammunition. I know my impact shooting the 33 gain JSB have quiet a good recoil shooting at low 800 fps, even the energy output must be lower. It never the less shoot good though:)
 
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Recoil? Take the same gun shooting the same bullet at the same velocity with different gun powders and you can have a difference in felt recoil. Powder burn rate. Relative quickness changes the dwell time of the bullet in the barrel, the flash out the barrel and the noise. Each one of those can change the felt or perceived recoil. I don't know much about air guns. Can air volume, and air pressure change getting the same velocity causing a variable? Interesting question. I am interested in the answers ahead.
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A short barrel using 4500 psi to make 100 fpe will have a LOT of more recoil than a long barrel using 3000 psi to make 100 fpe.
 
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The weight and momentum of your hammer will have some effect on perceived recoil, a bit like a scaled-down springer.
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I thought of that, but when dry firing there is litle felt recoil, the hammer are hitting with the same force as with a pellet in the gun. Also the valve will move forward, and shut again. But that is also felt litle when dryfire, so I think the movement of thouse parts has lilte effect on recoil.
 
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How much does the air in a average air gun plenum weight compared to a pellet?
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Hmm, really tough to answer this because if you mean how much air is released from the plenum compared to the pellets weight, that's entirely different than what the plenum holds, and plenum volumes vary greatly.



To give the best answer possible, I would say an efficiently tuned pcp shooting around 850-860~ fps, will likely release air that is 36%~ of the weight of the pellet. So when I shoot a 33.95 grain pellet, I use around 12 grains of air. Shooting a 25.4 grain pellet, I would need 9 grains of air. So many variables go into this as to give an exact average, but air weighing in at roughly 30-40% of the weight of your pellet is a good range for an efficiently tuned pcp.



The total weight of air in my plenum, well...it holds 53 cc's. At 2000 psi...the air density is 166 mg per cc at my temperature/elevation/pressure. So 53cc*166mg/cc= 8798 MG or 8.8 Grams or 135.7~ grains.
 
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"Also, the longer the barrel, the more length/time acceleration of said pellet can affect the gun/you"
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That sounds logic. My impact has the 700mm barrel, and the pellet will probably spend longer time in the barrel, as it is also traveling slower, than a .22 rim fire in comparison. So the "push" will last longer. 
 
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