How could you predict FPS changes by changing only pellet weight?

[dan_house]

My TX200 is shooting 8.64 pellets at 780 fps (55 degrees, 4500 ft ASL). Changing NOTHING else but pellet weight, is there a formula that would predict what speed, say, a 10.5 gr pel would be travelling?


Would save me a couple of evenings of chrony work....

TIA
dan

 

[Motorhead]

Only thing I can think of ( No calculator I've ever seen ) is take the Percentile % difference in weight and when shooting the heavier weight deduct that lighter pellets speed by the percentile of weight difference.

 

[PasadenaMike]

Use fpe as your common number (roughly) you can probably figure out your fps

 

[marflow 777]

your FPE should be close to the same so use that and the pellet weight and you could- would get FPS in the 5% + or- maybe

 

[airman_o]

this is the magic formula:

E = (kinetic) energy
m = mass
v = speed

it's not exactly exact because different pellets have different friction in the barrel etc, but it gets you in the right ballpark. When you're not changing the tune of your gun you can assume it produces the same energy with different pellet weights.

 

[Pea-shooter O.G.]

I can't add anything for a formula. I did run into a speed issue after a new spring was installed in my HW95. I wanted it sub 12fpe for a basic break barrel shoot with the Pistol class at a match and after 3 adjustments, I was still 730fps with JSB 10.34s. I was running low on time and patience so I tried the JTS 10.4s and I dropped to 700fps and the accuracy was still great. They seem a bit harder, so I think they have a bit more resistance running down the barrel. So, maybe try the JTS 8.7s and see if they give you good accuracy and slow you down a hair. Might be close enough that all your dope still aligns? My Marauder at 12fpe really likes the 8.7 JTS.

 

[Stubbers]

As PasedenaMike mentioned, you can use your current FPE as your base and calculate your estimated FPS from that with the new pellet weight.

New velocity = Sqrt( Original weight * original velocity ^2 / new weight)

New velocity = Sqrt(8.64 * 780^2 / 10.5) = 708 FPS

That gives roughly 708 FPS with the new pellet. However this is over simplified, but will get you in the ballpark.

-Matt

 

[Trucker3573]

Some predictions just stick in the head. 33.95 heavies run about 100 fps slower than 25.39. I think the same is true for 25.39 22 cal vs 18.1 gr. 18.1 gr are roughly 100 fps slower than 14.3. Rough numbers never exact but gets you a ballpark estimate.

 

[SilentSquirrel]

Watsa" matta'?
Don chu like to do no shootin" over the chrony?

 

[Paul Be Careful]

this is the magic formula:
View attachment 504347
E = (kinetic) energy
m = mass
v = speed

it's not exactly exact because different pellets have different friction in the barrel etc, but it gets you in the right ballpark. When you're not changing the tune of your gun you can assume it produces the same energy with different pellet weights.

That is the basic formula for kinetic energy and you are correct that it is relevant because it illustrates the connection between mass and velocity, but the ratio formula provides a more straightforward calculation for predicting velocity changes based on weight differences: v₂ = v₁ × √(m₁ / m₂)

This formula simplifies the calculation because it directly compares the two pellet weights without needing to calculate kinetic energy separately. By using the velocity of the one pellet and masses of the two pellets, it quickly shows how a change in weight affects speed, making it easier to predict the new velocity.

Getting geeky with it...

v₁: Initial velocity of the lighter pellet (8.64 grains) = 780 fps
m₁: Mass of the lighter pellet = 8.64 grains
m₂: Mass of the heavier pellet = 10.5 grains
v₂: Velocity of the heavier pellet (which we want to find)

Substituting the values into the formula:
v₂ = 780 × √(8.64 / 10.5)

Calculating the mass ratio:
8.64 / 10.5 ≈ 0.8219

Taking the square root:
√(0.8219) ≈ 0.9066

Calculating v₂:
v₂ = 780 × 0.9066 ≈ 707.15 fps

​

So, the velocity of the 10.5-grain pellet would be approximately 707.15 fps, assuming no other changes.

You're also completely right that velocities won't necessarily scale perfectly this way. Several factors influence performance, including the gun’s design (barrel, twist rate, choke, harmonics), the type of projectile, and air pressure—not just the pellet’s mass, as you have smartly pointed out.

 

[Moderator3]

Here is a link to a page full of airgun related calculation tools.

Airgun Calculators

https://www.airgunnation.com/calculators/

www.airgunnation.com

 

[Paul Be Careful]

As PasedenaMike mentioned, you can use your current FPE as your base and calculate your estimated FPS from that with the new pellet weight.

New velocity = Sqrt( Original weight * original velocity ^2 / new weight)

New velocity = Sqrt(8.64 * 780^2 / 10.5) = 708 FPS

That gives roughly 708 FPS with the new pellet. However this is over simplified, but will get you in the ballpark.

-Matt

I failed to see you nailed this before i chimed in. Kudos.

 

[Paul Be Careful]

Here is a link to a page full of airgun related calculation tools.

Airgun Calculators

https://www.airgunnation.com/calculators/

www.airgunnation.com

That’s a very useful set, but unless I’m missing something, I don’t see a calculator that can solve the original poster's question.

 

[Moderator3]

X weight pellets at Y speed =Z fpe.

You have two known variables, the calculator will solve for the third, unknown

Once you have the fpe of the gun, put in the new pellet weight and it will tell you the (approximate) new speed.

 

[JimD]

I agree assuming the fpe stays relatively constant is a good way to estimate. Accuracy might get a little better if you also assume a heavier pellet will tend to have a little higher fpe with the tune staying the same.

 

[JimNM]

I agree assuming the fpe stays relatively constant is a good way to estimate. Accuracy might get a little better if you also assume a heavier pellet will tend to have a little higher fpe with the tune staying the same.

I do think that the energy of the system will remain nearly constant across the weights of pellets we can choose from. If the gun is set to 22 FPE with a heavy pellet, it should be nearly exactly the same 22 FPE with a lighter pellet and no other changes made. You are trading speed for weight...but the ENERGY in is the same for both selections. Down range you may be able see a difference due to BC...but interior ballistics will hold true.

IMHO and I am married, so I am used to being wrong at least twice a day


Dadgum it... I am wrong. The question was FPS not FPE. My bad for reading comprehension error. Carry on!

 

[dan_house]

Thanks Guys!!

Lot of great info that I'll need to work thru a bit to understand ...

 

[dan_house]

Watsa" matta'?
Don chu like to do no shootin" over the chrony?

Dont mind at all shooting over the chrony, just not 250 times in an evening to sort some of this out.

did that once at the range...... paid for it the next couple of days