How much pressure do I need?

[rkr]

I've been planning a bottle conversion of my AR6 pistol. Nothing fancy really, a drop block after the original short tube and a 3000 psi paintball bottle with regulator and around 24" barrel. I would like to feed it with 44 grain 225438 bullets as I have a mold for them and they fit in the magazine. The question is what regulator setting do I need? I'd like to shoot those bullets at around 880-920 fps making 75-85 fpe. Higher speed would be fine of course but I'm not going for an "all out blow air long after the bullet is gone" tune. I can get fitting reg/bottle combos with 2000 psi and 2200psi settings from china and I'm wondering if 2000 psi would be enough for what I want?

[mackeral5]

This will be an interesting build.  I look forward to seeing how it goes.  What little experience I have with smaller bores and heavier slugs tells me you'll likely need more pressure than you initially think, possibly closer to 3000psi.....How much plenum do you expect to have? 

[rsterne]

I have a regulated PCP with 28" barrel that on 2800 psi does 1000 fps with a 45 gr. slug.... That is 100 FPE, with decent efficiency.... The gun has full bore-sized porting.... With a 24" barrel, I would expect to need about the same pressure to make 85 FPE, and around 2500 to make 75 FPE.... If your ports are smaller, you will need more pressure to get the same thing.... YMMV of course....

Bob

[rkr]

I have a 24" barrel .224 that makes 100 fpe with 47 grainers at 2900 psi (980 fps). Barrel size porting is not an issue with AR6 as it fires through the magazine. I was just wondering if there a spreadsheet of sorts that would give a good guesstimate on pressure. I guess I need to go 2200 then and perhaps consider hollow point conversion of that mold to get the bullet weight down a bit.

[mackeral5]

How much plenum?

[rsterne]

Are ALL the ports in the AR6 bore area or larger?.... Transfer ports & valve exhaust port need to be bore size, and the throat area should be at least 10% greater....

"Lofty goal" for FPE = barrel volume (CI) times pressure (psi) / 24.... ie for a 24" barrel, FPE = bore^2 x PI/4 x pressure.... This is half the theoretical maximum FPE, and allows for the mass of the air, and other typical losses....

It gives you a number hard to achieve, with bore-size porting and at maximum power (valve open until slug exits).... I usually only reach about 90-95% of that with my builds (assuming bullet weight = FPE/2), and then back off from there to get acceptable efficiency.... If your plenum is less than 1 cc / FPE, it gets even harder to approach this goal....

You can turn the formula around if you have a desired FPE to find the minimum required pressure.... Pressure = 24 x FPE / barrel volume.... or for a 24" barrel, Pressure = FPE / bore area.... Add about 10% at least to that number, unless you build better than I do.... 

Bob

[rkr]

OK, plenum is somewhere in between 60-70cc so almost there but not quite. The lofty goal for 24" barrel would be 2000 psi which means I want 2200 psi in real life. Ports are smaller now but I will fix them for max. power. It's almost doable but I'm afraid it will fall a bit short. Looks like I need a bit lighter ammo or longer barrel or higher pressure to meet my goals.

[rsterne]

Don't forget, the lofty goal is for a maximum power shot, which means an air hog.... If you are currently getting 100 FPE at 2900 psi, and are satisfied with the efficiency, and you want 80 FPE from the same gun but regulated.... then that means (0.8 x 2900) = 2320 psi....

Bob

[rkr]

Don't forget, the lofty goal is for a maximum power shot, which means an air hog.... If you are currently getting 100 FPE at 2900 psi, and are satisfied with the efficiency, and you want 80 FPE from the same gun but regulated.... then that means (0.8 x 2900) = 2320 psi....

Bob

You are right, although 75 fpe would work at 2200 psi. I still would like to have some margin for error though. I have a 26" 15.5" twist FX liner that I could try mounting on the gun, I think that might work with .218 sized bullets. The only problem is that the liner is 8mm in diameter and I need 10mm threads on the barrel to screw it to the breech ... Then there's the option of using .172 liner but bullet molds are not available 

[rsterne]

6mm is also a possibility.... The Arsenal 6mm "Bowman" is an excellent slug, and NOE have a couple, if they are in stock.... TJ's have a liner....

Bob

[rkr]

6mm is also a possibility.... The Arsenal 6mm "Bowman" is an excellent slug, and NOE have a couple, if they are in stock.... TJ's have a liner....

Bob

I already have the bowman mold in .224 so no point in going heavier as I'm mainly looking for good bc (comparable to .22lr) with decent air usage. Maybe I should wait a year or so for the .177 slugs to become main stream, that might be the answer for both of my goals.