How do I calculate how much air is in a plenum?

[MadCowCrazy2]

So then it would be: 0.049*4350*39/24=347

So a .25 cal shot with 4350 psi from a 39" barrel has an output of 347 foot pounds of energy to push the projectile with.
I found a FPE to FPS calculator on airgundepot.
It's probably not the correct way to calculate as I'm guessing the above math calculates the force pushing the projectile and not the fpe of the projectile?

Airgun Calculators | Airgun Depot

www.airgundepot.com

 

[sb327]

For Bobs goal to be reasonable, a projectile weight of half (in grains) of desired fpe (or larger) is needed.

Dave

 

[AlanMcD]

"So a .25 cal shot with 4350 psi from a 39" barrel has an output of 347 foot pounds of energy to push the projectile with."

No - that is a theoretical maximum energy one could get into the projectile by the time it leaves the barrel, not the force pushing on it. And it is not a design rule or such, but a means of looking at potential capabilities, and the harder one pushes things the less applicable it will be. If trying to make the most power out of a given caliber, the best one will do will far fall short of the theoretical maximum for that configuration. Lower the pressure to 3000 or so and things become more reasonable as an estimate, in no small part as compressed air does not behave like an ideal gas once one goes above about 3400 psi due to Van der Waal forces (that we ignore at lower pressures because we can do so reasonably).

The maximum force on the projectile to accelerate it would be about 213 pounds of force, and it would start to drop as the projectile accelerates in the barrel.

Honestly, you keep misapplying concepts here . . . not sure what you are after, but it starting to seem like you want to make something currently unattainable due to basic physics . . .