Energy variations from a rifle, depending on pellet weight

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DHart

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Jul 7, 2018
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AZ, United States
I've gotten the impression that some rifles may develop more energy from a lighter pellet than from a heavier pellet - is this generally true? Or can energy from a given rifle generally be expected to be about the same when going from a lighter pellet to a heavier pellet?

Obviously, a chronograph and a few calculations will easily answer this question, but lacking a chronograph, I'm wondering if a general estimation of velocity (then used to calculate energy) can be made for different pellet weights, if one knows the velocity for one pellet weight?

Let's take a .22 caliber, for example.

If a PCP gun moves a 15.89 gr JSB pellet at 911 fps for 29 fpe, can a reasonable estimate (lacking a chronograph) be made for what the velocity/energy would be with a change to

18.13 gr pellet?

21.14 gr pellet?

25.39 gr pellet?


 
This is the formula to calculate ft. lbs. of energy (weight X velocity X velocity / 450240). Generally speaking you will get more energy from a heavier pellet up until diminishing marginal returns start to occur. Once the weight of the pellet is too great the rifle won’t be able to push it fast enough and the energy will decrease. In Glems example above, anything heavier than the 31 grain ammo will most likely start to show a decrease in energy the heavier the pellets get.
 
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My guess for an estimate would be:

(weight of light pellet) / (weight of heavier) x fps

All I am doing is getting the percentage weight difference of the pellets and adjusting the fps by that percentage.

That just assumes one thing though: The same amount of pressure in the same length tube effects the movement of the 2 weights the same. That is incorrect in the real world and I have no idea what percentage that varies the estimate. Would be a good test to run.
 
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Generally, when comparing a light and heavy pellets, the heavier pellet with a slower velocity(to a degree) produces more energy. Here's an example using the information gathered from shooting my .25 caliber with my 2 favorite pellets, that will show what I mean...

25.4gr @ 935fps = 34.97lbs.

33.95gr @ 830fps = 40.62lbs

No adjustments to the gun were made. I shoot the 25.4gr pellets at 935fps and then stick in a 33.95gr pellet and get 830fps. Even with less velocity, heavier pellets not only deliver more energy but depending on design almost always a higher BC's too. As Olevey stated above, diminishing returns... at a certain point, if the pellet is moving too slow, the mass of the pellet cannot make up for the loss of velocity to create the energy gains. Stoti
 
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Generally, when comparing a light and heavy pellets, the heavier pellet with a slower velocity produces more energy. Here's an example using the information gathered from shooting my .25 caliber that will show what I mean...

25.4gr @ 935fps = 34.97lbs.

33.95gr @ 830fps = 40.62lbs

No adjustments to the gun were made. I shoot the 25.4gr pellets at 935fps and then stick in a 33.95gr pellet and get 830fps. Even with less velocity, heavier pellets not only deliver more energy but depending on design almost always a higher BC's too. As Olevey stated above, diminishing returns... at a certain point, if the pellet is moving too slow, the mass of the pellet cannot make up for the loss of velocity to create the energy gains. Stoti
Click to expand...


Shows my estimate is way off ;)
 
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E=MC2 Energy= Mass x Velocity squared. 

Chairgun Pro does all of this stuff for you, it's a wealth of information. Without having the velocities of the different pellets, it's just a guessing game trying to figure out the energy of the 3 different pellets. I don't use springers enough anymore to even guess at the losses in velocity from increasing the pellet weights. 
 
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Toss this in excel and change the pellet weight (15.89) around...the (.0145/.22) is a port reduction correction factor, put your port size there...the (19.5/24) = barrel length so replace 19.5 with barrel length, the 1900 is avg pressure at valve seat...the *.90 Correction factor in equation is for pellet mass to air mass ratio, the number would go UP the heavier the pellet, and down the lighter...I did not go to that length in this example I left the ratio the same.



=ROUNDDOWN(SQRT((ROUNDDOWN(((15.83*2)+ROUNDDOWN((0.22^2 * 3.14159/4) * 1850 * (19.5 / 24) * 0.13/0.22,3))/2,2)*450240)*0.85/15.83))

the example above gives me 915 FPS with 15.89. Which is 29.5 FPE

=ROUNDDOWN(SQRT((ROUNDDOWN(((18.13*2)+ROUNDDOWN((0.22^2 * 3.14159/4) * 1850 * (19.5 / 24) * 0.13/0.22,3))/2,2)*450240)*0.9/18.13))

If I change the pellet weight ONLY to 18.13 I get 884 FPS. Which is 31 FPE. Roughly 2 FPE Gain. 

=ROUNDDOWN(SQRT((ROUNDDOWN(((25.4*2)+ROUNDDOWN((0.22^2 * 3.14159/4) * 1850 * (19.5 / 24) * 0.13/0.22,3))/2,2)*450240)*0.9/25.4))

If I change the weight to 25.4 I get 821 FPS @ 38 FPE 9 FPE Gain over 15.89 



Those numbers will likely change once you put in your actual numbers (port size/barrel length/pressure)



The above formula requires one additional correction factor that I use to be more accurate but its a pain in the butt to implement so I will leave it out...the above will give you a really close idea of what happens with pellet weight changes...but the lighter you go on pellet the less likely you will reach that number due to the lack of my additional Correction Factor...also the pellet to weight ratio factor changes with each weight which I did NOT provide in this example because I am too lazy to do the math and don't even know the mass of air you use per shot based on your volumetric efficiency...



Typically what happens with lighter pellets, the lighter you go the less of the Pressure Gradient applies to them due to their acceleration...the heavier pellets both move slower so have more KE transfered to them on TOP of having a higher pellet/air weight ratio...hope that makes sense.



-Matt
 
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E=MC2 Energy= Mass x Velocity squared. 

Chairgun Pro does all of this stuff for you, it's a wealth of information. Without having the velocities of the different pellets, it's just a guessing game trying to figure out the energy of the 3 different pellets. I don't use springers enough anymore to even guess at the losses in velocity from increasing the pellet weights.
Click to expand...



Would be nice if you could use the above formula to conclude our pellets terminal velocity at barrel exit...but the truth is neither that nor F=MA (force = mass*accel) combined alone will get you an answer, its much more complex...here is a set of actual equations to determine what happens in a pcp...Courtesy of Domingo Tavella...pretty sure I don't see f=ma or e=mc2 in their basic format..




 
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I bet you would get 33 FPE or so with the 25.4. Around 780 or so fps.
Click to expand...

That depends on his rifles configuration...

For example if I go from a 19.91gr to a 33.95gr (.25 cal 42% weight increase) which is a bit more of a stretch than 15.83 to 25.4 (38% weight increase) but very similar....I go from roughly max FPE of 48~ @ 19.91 pellet to 58~ on the 33.95...nearly 20% gain in power, so for his to go from potentially 29.5 to 38 would be 22% gain in power...thats approximated with no rifle config details and at max power.


As I said my formula for me would be set up 100% to my gun configuration(port/pressure/barrellength) and would be for MAX power. If I detuned/ran less hammer spring for say only 25 fpe on the 19.91 and go up to 33.95 my gain % is smaller, but still considerable, from 25 fpe to 28.2 ( 12% )..so as you see the gains become less the further down your plateau you run which is another Correction factor I include but its rather complex....


So essentially, the less you open your valve (the lower down the plateau) the less of the % change you will see. The formulas I posted are for MAX FPE comparisons



The Correction Factor that is TUNE specific as opposed to MAX FPE specific would be added with the following... ValveLift/ValveLiftMax <---I doubt many are willing to figure these out...

Valve lift stands for how much lift you obtained on that tune.

Valve lift max stands for the lift required to obtain max flow value of your ports..

Valve lift = hammer kinetic energy / closing force

VLiftMax = 1/4 Throat Diamater after accounting for the area the Valve stem takes...



This would only apply if VLIFT is UNDER VLIFTMAX value...otherwise you just use the MAX FPE specific formula...or use a simple IF statement to check if the vliftmax value was exceeded.



Rabbit hole gets deeper and deeper the more accurate you wanna be unfortunately, to the tune of those ordinary non-linear differential equations...my formula is a copy and paste and replace certain rifle specific factors with your own (pressure/barrel length/port) being most important.
 
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My guess was assuming he's maxing out at 27FPE with the 15.89's. Really just a guess, not knowing which gun or if he's even maxed out etc.
Click to expand...

The gun is a Bantam Sniper HR .22. Regulator is set at about 150 bar and not sure where the hammer spring is set, but it was set up at AoA. Giving velocity of 911 fps with 15.89 gr. JSB, for energy of 29 fpe.

I'm pretty sure there is more that the rifle is capable of with tuning, but was wondering -if I change nothing but pellet weight- what velocity I would likely get with 18.13 gr, 21.14 gr, and 25.39 gr. pellets.
 
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