FX Velocity projection formula given a known weight and speed

[killraven4334]

Hi There,

I was hoping someone would be able to help me out. I am trying to reduce unnecessary spending and testing time and would like to get a rough idea of what velocities I could expect given a change in projectile weight only. No other changes. Say I reach 1000fps with 26grains .25cal projectile, what would the formula be determine the velocity of a different weight projectile?

 

[beerthief]

Hi There,

I was hoping someone would be able to help me out. I am trying to reduce unnecessary spending and testing time and would like to get a rough idea of what velocities I could expect given a change in projectile weight only. No other changes. Say I reach 1000fps with 26grains .25cal projectile, what would the formula be determine the velocity of a different weight projectile?

PyramydAir Airgun Calculator: Muzzle Energy

Calculate power output, muzzle energy and more! Whether you need an FPE Calculator, Ballistic Coefficient, or Wind Drift with Pyramyd AIR's shooting calculators

www.pyramydair.com

 

[Superpete]

ShootersCalculator.com | Ballistic Trajectory Calculator

Calculates the ballistic trajectory of a bullet fired from a rifle, handgun or other firearm. Produces a ballistic trajectory chart and table that shows the drop, velocity, kinetic energy, windage, and trajectory of a bullet.

shooterscalculator.com

 

[Raider03]

As a first cut, calculate the FPE for the current weight and speed with a ballistic calculator, like the one beerthief provided. Then enter the new pellet weight and adjust the velocity until you get the same FPE. For lighter pellets this velocity will be slightly high as lighter pellets won't be as efficient. Heavier pellets will be more efficient, so this estimated speed will be lower then actual. This relationship breaks down at some point as weigh increases because the efficiency improvement platues.
Keep track of the error between the constant FPE estimate and the actual FPE, you can use that correction for future estimates, relative to the first speed, weight data.

 

[subscriber]

A simple conversion that hold the energy constant is a fair starting point. To refine it, the fact that PCPs are often more efficient with heavier projectiles needs to be taken into account.

So, if you start with a light fast know velocity projectile, then a heavier one will tend to go slightly faster than the simple constant energy calculation predicts. Starting with a know velocity of a slower heavy projectile will tend to overestimate the velocity of a lighter one.

This assumes near constant friction. If the slugs have bearing diameters that are larger and longer, that tends to reduce velocity. Ditto for harder lead alloys.

Your 25 grain .25 projectile suggests a pellet. If you are shooting pellets, the friction aspect is much smaller than it would be for slugs. But the efficiency improvement seen with heavier projectiles still applies. Efficiency of PCP airguns suffers when you try to shoot projectile closer to the speed of sound. A 25 grain pellet at 1000 FPS carries 55.5 FPE at the muzzle. A 34 grain pellet at 858 FPS carries 55.8 FPE. It is likely that if your 25 grain pellet has a measured velocity of 1000 FPE, that a similar diameter 34 grain pellet of similar alloy would shoot over 858 FPS. How much over is likely to depend on the barrel length and type of valve system, but it should be more than measurement error.

 

[Raider03]

As a first cut, calculate the FPE for the current weight and speed with a ballistic calculator, like the one beerthief provided. Then enter the new pellet weight and adjust the velocity until you get the same FPE. For lighter pellets this velocity will be slightly high as lighter pellets won't be as efficient. Heavier pellets will be more efficient, so this estimated speed will be lower then actual. This relationship breaks down at some point as weigh increases because the efficiency improvement platues.
Keep track of the error between the constant FPE estimate and the actual FPE, you can use that correction for future estimates, relative to the first speed, weight data.

Hi, attached is data from my Crown. I hope it helps. I calculated the FPE ratio using the H&N 20gr pellet because they were the lightest lead pellets.