Dual Tank Benchrest Rifle

[rsterne]

I have come up with a crazy idea for what may be my last big (and heavy) project.... My hands are getting very arthritic, and I don't know if I will be able to complete this one, but I have decided to have a go.... The concept is to use two 700 cc Carbon Fibre 4500 psi Bottles mounted under a plenum of equal length (16") made from 4130 CrMoly tubing of the same dimensions as an MRod tube.... ie 1.25" OD x 0.95" wall.... Here are the bottles, which I have on order from Wes at ArcheryAirgunFun in Saskatchewan.... https://airgunarcheryfun.ca/



The two tanks will be mounted side by side with the plenum above, using a scaled up version of this tank block I made for two 88 gr. CO2 cylinders.... The three holes in the block will be threaded 18mm x 1.5mm, with the regulator screwing into the upper hole, and stubs made from 18mm steel bolts used to mount the tanks.... They will be sealed to the block with an O-ring at the parting face to the top of the tank....



There will be a short piece of Picatinny rail mounted on the bottom of the block to mount a BiPod, like I did on the converted QB....



Here is a sketch of how I plan to mount an adjustable Ninja Regulator, with the bonnet inside the end of the plenum tube.... The adapter is shown in red....



I will turn down the OD of the bonnet by 0.020" for a length of 1/2", and that part will fit inside the end of the tube.... The bonnet setscrews will be accessible without removing the bonnet from the tube if a regulator rebuild is necessary.... Since the adapter is mounted with four 10-32 screws into the tube, the assembly can be turned in 90 degree increments for orientation of the regulator, and then the position fine tuned by rotating it on the 1/2" NPS bonnet threads, and then the position locked into place with two setscrews.... Here is what the dual tank CO2 setup looked like to give you an idea of the plan....



I have not made a final decision yet on caliber, but if Al at NOE makes the mold I have asked for, this Lothar Walther barrel is what I would use....

https://www.lothar-walther.com/gun-barrel-blanks/rifle-barrels/standard-european/1079/barrel-blank-twist-5.9-4-6x30mm-od-1.26-l-26.18-cr-moly-steel?c=34





I haven't decided whether to use this profile, or something a bit lighter....



Here is a drawing of the bullet I have asked Al to make a mold for.... It is a full 4 calibers long, and made for the 5.9" twist of the 4.6mm barrel from LW....



This is very much a matter of finding the fastest twist barrel I could afford in a small caliber, and building the bullet and gun around it.... It will take about 3000 psi to get into the 900's and the plenum is good for the full 4500 psi at a 3.5:1 safety margin.... There will be 1.4 litres of air (85 CI), so if I can achieve an efficiency of 1.0 FPE/CI, at 85 FPE I should get about 70 shots with a 1000 psi (70 bar) drop in tank pressure.... I hope it all comes together.... 

Bob

[bear air]

Sounds like fun Bob. I really like the double block. I have a 30 cal armada I wouldn't mind double tanking.

[sb327]

I think it’ll come together just fine. For sure will be interesting. Look forward to progress updates.

Dave

[customcutter]

Following! 

Time to start those winter projects for folks up north.  Too cold to do anything outside.  Here in central Fl, it'll be cool enough to work in the garage for more than a couple of hours each morning.

[rsterne]

Pending finding a regulator that can handle outputs of 2800-3600 psi that will fit and I can afford.... and considering the costs.... this project is unfortunately on hold for now.... 

Bob

[sb327]

What about the pcp only one ninja makes? It has a steel output thread and I think a smaller diameter on the high side of piston. I can check that if you want me to.

It comes set at 3000 but I’m sure you could change Belleville stack.

Dave

[Scotchmo]

There are 3000psi regulators available. 3000psi is a good set point for a high energy airgun.

Adding a couple inches of barrel length could get you the same fpe potential as adding a couple hundred psi.

It’s an interesting project. 1400cc on a .177 bench - that’s a lot of shots.

[rsterne]

I have been looking at the numbers, and I think I will need more than 3000 psi to get into the 1000's.... The barrel is only available in one length, 26".... I don't know if the high pressure Ninja can go to 3600-3700, but I think 3000 is about the minimum usable with such a high SD slug (0.166) with a 26" barrel....

Bob

[Scotchmo]

I put the design into my spreadsheet and got these results:

3000psi set pressure
215cc plenum
0.18 caliber with full diameter ports
1400cc tank filled to 4000psi
stock Cothran Powerhouse Marauder valve
26" barrel
assume a SSG type or solenoid activated hammer, very light, giving a 0.0025 sec dwell

39gr slug
1028fps 91fpe
73 shots per fill

The spreadsheet results are usually fairly close to actual results, so you should be able to get pretty close to 1000fps.

For maximum FPE with the 26" barrel at 3000psi set point, if you could extend the dwell to 0.0035 (anymore and you risk blasting out lots of excess air without any gain):
1090fps 103fpe
43 shots/fill

[sb327]

I just checked the regulator. I can get it set to approx 3900 by adding 1 Belleville to the stack. That’s pretty close to max I feel like.

The only thing about these regs are they are normally only found in 5/8-18. Hmm, just a thought, maybe the bodies are the same as normal regs, I’ll have to check that.

Anyway, so yes, you can run the pressure up on them.

Dave

[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

[sb327]

Large side piston (cylinder actually) .390”
Small side cylinder .319”

This is on the fixed output 3000psi steel bonnet

I was mistaken on my ninja set flex, it’s a 1700-2200, not a 3000.

It has the same small side piston and the large end cylinder is .549”

Hope that helps.

Dave

Flex on the left, obviously

[Scotchmo]

OK. I do it a little different. I do 100% accounting for energy:

energy in = energy out

potential energy of air used = slug FPE + sliding friction(heat) + KE of air + remaining potential energy of air

I've been using very long barrels, so getting the desired FPE mostly becomes a function of adequate dwell. I guess my fudge factor is the dwell. Underestimated dwell would throw off my shot count. The best way for me to account for efficiency would be:

(actual shot count)/(expected shot count)

With a big tank volume, shot count is rarely an issue. OK, I now seem to remember us having this conversation before.

I still think with 3000psi, you can get the 1020fps with sufficient dwell (probably not the 1090fps). After EBR, I'll play with the Cothran valve dwell and see if I'm really off.

What kind of valve do you plan on using?

[rsterne]

I was planning on making my own balanced valve, with a change or two to incorporate what we all have learned about them over the past couple of years.... Full size porting goes without saying, and I use a retractable bolt probe that is adjustable so that I can occlude the barrel port to reduce power if required.... The combination of optimizing the SSG hammer for the pressure used, to get decent efficiency and a low ES, and then the ability to choke down the flow to the bullet is a good one that I have used successfully for several years now....

I have been thinking about what you were saying about using a Flex and then changing to the HP Ninja for increased stability, and that sounds great if you are working with under 3000 psi (within the range of the Flex).... I can do something similar to determine the setpoint of the regulator by using a "tiny tank" on the regulator input and tethering it to my SCBA while experimenting with the pressure.... I did that with my 6mm.... Click on the pic to enlarge....



I have two of the tiny tanks for regulator testing, one 5/8"-18 and the other 18x1.5 mm.... They only have a volume of a few ccs.... In that photo I have a digital gauge on the regulator inlet, and I can feed that with a tether for continuous shooting (at whatever the reg. setpoint is) or I can shut off the SCBA tank and bleed the line to determine the pressure in the plenum (block and tiny tank included) for one shot....

The new gun would be the same basic design as the 6mm above, but the regulator would be inline with the plenum, attached to the top of the "Y" block holding the two bottles....

The reason I was a bit confused on the Ninja HP regulator is that I have one here, that says 2200 psi on it, and "not for paintball", and it looks just like yours, Dave.... However, the piston is the old 0.55" diameter one, and to get 2200 psi it has four sets of triple 0.032" Bellevilles and some thin shims.... They are arranged.... ))) ((( ))) (((

I have a spreadsheet that calculates the force needed from the Bellevilles for any given pressure output for the 0.55" piston regs.... I copied it and changed the large end to 0.390", and then looked at what the setpoint might be with 10 and 11 Bellevilles that are 0.032" thick (with no shims).... I got the following, assuming that the space available for the Bellevilles (0.430") is the same as the regs. with the larger pistion:

10 x 0.032" (no shims) = 2150 psi @ 91 lbf..... and varying by 43 psi per thou, with room for 0.110" of shims....

11 x 0.032" (no shims) = 3800 psi @ 161 lbf..... and varying by 39 psi per thou, with room for 0.078" of shims....

In both cases, there is room for more shims than required to drive the 0.032" Bellevilles completely flat, which requires 260 lbf.... That would give a setpoint of way over 4500 psi, so that means that if the room for the Bellevilles is still 0.430", the stack of 10 x 0.032" should be adjustable from 2200 psi to tank pressure, and with 11 x 0.032" from 3800 to tank pressure.... If these assumptions and calculations are correct, that stack of 10 x 0.032" is perfect for the Ninja HP reg.... A 0.030" shim should give about 3400-3500 psi....

Bob

[JuryRigger]

Watching with interest, that many shots at that FPE will be a beastly build! 
Jesse

[sb327]

Just for information purposes Bob. I had an idea on my flex that seems to work.

I made a piston with a smaller dominant head and a sleeve for the bonnet. The sleeve is held in place by a steel washer. It makes the bonnet set up just a tad higher but doesn’t cause any issues.

The Belleville stack is set up as singles now and appears to work as intended.

Dave