OK, so now you know what the theoretical MAXIMUM FPE possible is.... The big question is, what are the losses, and what percentage of that number might you actually be able to obtain, if you do a really good job?.... I'll give you the simple answer first, and then try and explain why.... If you can get 50% of the theoretical maximum FPE on air.... or 70-75% on Helium.... you have done a great job !!!

The biggest single loss in the system is accelerating the mass of the air (gas) itself.... Let's go back to our first example, a .25 cal with a 24" barrel at 3000 psi.... The mass of the air in that barrel is the barrel volume times the air density at 3000 psi.... The volume is (.25)^2 x PI/4 x 24 = 1.18 CI, and air at 3000 psi has a density of 238.6 kg/m^3.... That converts to 60.3 gr./in^3, so the air in the barrel weighs 71 grains, and somewhere about half that total mass will be travelling at the same muzzle velocity as the bullet....

.... Helium at 3000 psi has a density of 31.0 kg/m^3, which is only 7.84 gr./in^3, so the same barrel full of 3000 psi Helium would only weigh 9.2 grains.... only 13% the weight of the same amount of air.... This is the primary reason that you can get a LOT more power by using Helium.... Much less of the available FPE is lost accelerating the mass of the gas, so more is available to accelerate the bullet.... It's as simple as that....

If you have been following along so far, it may have occurred to you that the heavier the bullet, the smaller the percentage of that is the mass of the air.... This means that heavy bullets can extract a greater percentage of the maximum available FPE.... Therefore, to be able to come up with a percentage number that has a chance of working, we have to "fix" the weight of the bullet, in relationship to the FPE available to accelerate it.... Since if a bullet is travelling 950 fps, the FPE is twice the weight in grains, I chose that as a standard.... After looking at a LOT of high performance PCPs, I have found that the best of them can approach 45-50% of the maximum theoretical FPE on air, when tuned to shoot flat-out.... I don't have enough data on Helium to give you a firm rule of thumb, but about 50% higher FPE with Helium than with air seems reachable, from the small sampling of PCPs we have using Helium.... which would make the percentage using Helium about 70-75% of the maximum....

Let's go back to our example, where we calculated the maximum FPE as 294 FPE.... 50% of that is 147 FPE, achieved by pushing a ( 147 / 2 ) = 73.5 gr. bullet at 950 fps.... If you can do that with a 24" barrel, on 3000 psi, you have done a great job designing and building the gun.... I have never quite made it.... It always takes me a few inches extra barrel length, a bit more pressure, or a heavier bullet, to get to that 50% level.... So, if you manage it, pat yourself on the back for a job well done.... If you think about where the extra (50% of the) energy has gone, remember that about 71 FPE went into accelerating the air, so that leaves us with about ( 294 - 147 - 71 ) = 76 FPE (roughly 25%) of losses to friction, etc.etc.... I guess if you assume the same 76 FPE of losses in a gun running Helium, that might leave you with (294 - 9 - 76 ) = 209 FPE available for the bullet.... which is 71% of the theoretical maximum FPE.... as good a number as any to use for an estimate for using Helium....

OK, to summarize.... For our example, a .25 cal with a 24" barrel running on 3000 psi of air, the theoretical maximum energy is 294 FPE.... About 25% of that is lost to friction and other losses, another 25% goes into accelerating the air, and there is about 50% remaining to propel the bullet.... You shouldn't consider this as a "hard number", but as a "lofty goal".... one that if you can achieve, you can be very proud of.... Taking this 50% into account, changes the formula to the following....

**FPE Goal (air) = Bore area (sq.in) x Pressure (psi) x Barrel length (in.) / 24** .... where the constant you divide by, 24, changes inches to feet, and includes the 50% factor....

Bob