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All Springer/NP/PCP Air Gun Discussion General > "Bob and Lloyds Workshop"

Dual Tank Benchrest Rifle

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rsterne:
If I input the data from my .172 cal at 2800 psi (165 cc plenum) with a 28" barrel, where I get 1050 fps with a 24.5 gr. slug with an efficiency of 0.96 FPE/CI, using a dwell of 0.00243 sec.... Lloyd's spreadsheet says the overall efficiency is just 79%.... If I then change to .183 cal and 39 gr., increase the plenum to 215 cc at 3000 psi, with a 26" barrel, using a dwell of 0.0025 sec. and staying with the 79% efficiency, I get only 923 fps (74 FPE) with an efficiency of 1.12 FPE/CI.... It would require an increase in efficiency to 90%, and a dwell of 0.0027 sec. to get your predicted velocity and shot count.... I think that is extremely unlikely.... If I drop the efficiency down to 80% (I think much more likely), leaving the dwell at 0.0025 sec. I then get 1020 fps (90 FPE) at 0.98 FPE/CI.... Increasing the dwell to 0.0035 sec. (valve closing at the muzzle), I get 1070 fps (99 FPE), but the efficiency drops to 0.61 FPE/CI.... I think basing these numbers off an existing .172 cal gun I already have should be closer to reality, so I would not count on being able to use much less than 3600 psi or so to get into the mid-1000's....

Bob

rsterne:
Hey, Dave.... That's really good information, many thanks for testing it for me.... What are the diameters of the ends of the piston (where the O-rings are)?.... Actually the ID of the bores would be the most accurate information.... Using the original 10 x 0.032" Bellevilles and shimming them would give a wonderful range of adjustment (actually should be able to get to about 4300 psi before they go flat, 10 Bellevilles have a higher spring rate than do 11, with the same maximum force).... The threads are not an issue, I can thread the top of the tank block whatever I want.... However, I would have to drain and disassemble the gun to adjust the regulator by shimming.... Not a complete deal breaker, but I was hoping to use the adjustable Ninja, with the wheel for external adjustment.... I wonder if they use the smaller diameter piston?....

Bob

sb327:
The one I got from haji (flex) had the larger size piston head. It was set up for 3000 max by him. I have another set up from ninja (flex) that is a small bottle thread and 3000 max, I can check it.

Next time I’m in the shop I’ll get some more info for you.

Dave

Scotchmo:
Bob,
You said:
"overall efficiency is just 79%"

"It would require an increase in efficiency to 90%"

What does the efficiency represent? Projectile fpe/???

The custom set point regulators that Hajimoto sells have M18x1.5 tank threads, and a steel bonnet.
https://hajimotoproductions.com/index.php/product/ninja-high-pressure-metric-m18-custom-output/

The Ninja-flex reg has an aluminum bonnet (at least mine does).

Except for the Flex, Ninja PCP regs with a 3000psi output rating have steel bonnets. At least the ones that I have encountered.

I have found that the custom regs seem to provide a more consistent pressure. I'll sometimes use the Flex reg to decide on a pressure setting, and once decided, then switch it out for a fixed pressure reg.

rsterne:
I thought you would remember, the "efficiency" in Lloyd's spreadsheet is a "fudge factor" to make the actual performance of the gun match the spreadsheet numbers.... Once you input the known data, there is only ONE combination of dwell and "efficiency" that will give BOTH the FPE and FPE/CI numbers you got with your gun.... Generally, the fudge factor that works for one gun is correct for a very similar one when you are trying to make a prediction.... His spreadsheet includes (or can ignore) the mass of air used, and a few other losses (transfer port volume, an estimate of bullet starting and sliding forces, etc.), but yes, it would be how much of a percentage of the "remaining theoretical FPE" (once those factors are included) ends up in the projectile.... Typical values are 75-85%, and occasionally higher or lower....

Thanks for the info about the regs....

Bob

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