Estimated power change with Caliber change?

[Blutroop]

I have a .25 prod I’m fixing to switch over to .172 when the last pieces show up. As tuned by previous owner it should be shooting .25 jsb 25.39 gr pellets at 850-880. Does anyone have a idea how these numbers should change when switched to .172 24-26gr slugs?

[Motorhead]

P-rod as in a pistol ?
What length .25 barrel on it ?

What length 172" barrel

[Blutroop]

Both barrels are 26”

[Motorhead]

WAG .... due to the much lower area value the pressure has to work against .... Mid to upper 700's

[rsterne]

Roughly speaking, FPE is proportional to bore area, which is proportional to the caliber squared.... So going from a .25 cal to a .172 cal, with the same barrel length and pressure.... could drop the FPE by (0.172/ 0.250)^2 = 0.47.... so roughly half the FPE in .172 cal.... This is for maximum power, with optimum bullet weight for each caliber, however.... so your results could be quite different.... The theory is, therefore, that your velocity would be (.172/.25) x 880 = 605 fps with the same weight bullet.... I would be shocked if you could push it to the high 700s in .172 cal.... without other changes....

Bob

[Blutroop]

The shot string was around 2000. If I go up to 3000 and port another valve closer to .172 then what should I expect?

[rsterne]

Too many changes to make a prediction.... "More" would be an appropriate answer.... You haven't even mentioned barrel length which is just as important as the pressure.... Tofazfou was getting about 950 fps with 26 gr. bullets out of a modded MRod with 28" barrel at 3000 psi.... I can get 1000 fps with the 24 gr. Hollowpoints at 2800 psi regulated, with my custom PCP.... The power is possible, it just depends on how many mods you want to make....

Bob