9mm PCP Carbine

[rsterne]

For the last few weeks, on and off while thinking about my Hayabusa project, I have been working on a 9mm Carbine.... It is my intention to set this gun up as a non-PAL rifle shooting 77.8 gr. EunJin pellets at just under 500 fps (~42 FPE).... As with a lot of my projects, it's just to learn what is possible and/or practical.... I used a Disco valve with the throat of the valve bored out to 1/4" and the exhaust port milled to 3/16" on a 20* angle.... The inlet side of the valve is streamlined to increase flow into the valve.... The transfer port is a piece of 1/4" OD x 7/32" ID brass tubing fitting into milled recesses in the valve and barrel and the breech is sealed to the valve with O-rings.... I machined the breech from aluminum, along with the barrel band and RVA.... I used a Disco fill fitting and cover.... The barrel band features a shouldered hole containing an O-ring so that the barrel is semi-floated....  

The barrel is a 9mm barrel liner (1/2" OD) from Track of the Wolf.... I had quite a time getting the pellets to feed, and it required a long taper to basically "size" the pellets as they are fed into the rifling.... I blued the barrel after doing all the machining.... The hammer is a stock 2260 unit with a longer cocking pin.... and the hammer spring is from a Disco.... Once I finish tinkering, I will be fitting a stock (ie non-adjustable) 2260 rear plug and trimming the hammer spring to keep the strongest shot under 500 fps.... The steel bolt uses a probe to push the pellet past the transfer port, and a 10-32 bolt as a handle, fitted with an oversize brass knob.... I tried a flow-through bolt, but pushing on the skirt of the pellet damaged them, actually tending to shear off the edge while seating the pellet.... The probe pushing on the center of the pellet allows the skirt to collapse slightly while seating, avoiding this problem....  



I filled the gun to 2000 psi and started testing.... To my surprise, the peak of the velocity curve is at very low pressure, even with the hammer spring near coil bind.... As you back off on the hammer spring preload, you have to start at a lower fill pressure.... and the peak velocity occurs at an even lower pressure.... By the time I found a non-PAL setting, I was only filling to 1400 psi.... The velocity was peaking at only about 900 psi.... and in all cases, the last shot in the string (where the velocity was down ~10% from the peak) was at under 600 psi.... with the pressure reading about 400 psi at the end of all the strings....  



The efficiency is very high.... At maximum, the gun is delivering ~52FPE for 10 shots (514 FPE total) at 1.2 FPE/ci.... Two turns out on the adjuster, it is getting ~46 FPE (508 FPE total) over 11 shots for 1.4 FPE/ci.... At 4.5 turns out, it is returning ~40 FPE (356 FPE total) for 9 shots, again at 1.4 FPE/ci.... The limited number of shots is due, of course, to the relative small reservoir.... but it compares favourably to my .22 cal PCP Carbine Grouse Gun which gets 16 shots at 20 FPE from the same size tube (320 FPE total), for about 1.2 FPE/ci.... I may try restricting the transfer port to see if I can flatten the shot string for the non-PAL version and get more shots.... It should be possible to do so.... If not, I will likely use a regulated 13ci tank and shorten the stock....  

Bob

[rsterne]

I know, I'm replying to my own topic....  .... This thread is copy/pasted from another forum because I wanted to use my latest project by way of introduction here.... I have added the information below to the thread already, so here it is....

Well, I had some time this afternoon to make a new transfer port from a piece of 1/4" brass rod.... I started at 1/8" bore which was too small, but showed I was on the right track.... I drilled it out to 9/64" (0.140") which just happens to be the same size as a stock Crosman TP.... This allowed me to crank in the hammer spring to just under 2 turns out, and get a flatter shot string.... The black line in the graph below is what it did previously in the non-PAL setup.... while the red line is the way it works now.... 



By the time I was finished, I ended up with a longer string (12 shots) that was much flatter (8% variation).... The average velocity was 487 fps with a low of 460 and a high of 499.... Average energy was 41 FPE for a total of 491 FPE over the string.... The starting pressure was now 2000 psi and the reservoir was down to 700 psi after the last shot.... That meant the efficiency was even better at 1.46 FPE/ci.... 

I'm only shooting inside, but the entire string goes into one ragged hole.... This gun is pretty much done until I can get outside in the Spring and do more testing at longer ranges.... I think the idea of having a non-PAL gun that puts out 41 FPE is pretty cool.... 

Bob

[Rescue35]

Welcome and thanks for the information. This will help me a lot in my build. I'm currently working on a .45 and 9mm rifle. In fact I'm heading back to the lathe to finish working on the valve body. Keep us updated.

[rsterne]

I've just swapped out the hammer spring for a stiffer one (0.040" wire instead of 0.035").... I'm out of EunJins but more on the way.... Once I get my pellets, I'll be trying to see what I can get for maximum performance before detuning it for Canada's wonderful under 500 fps limit.... I have my PAL (Possession and Acquisition License) but this gun I don't want to have to register....

Bob

[lloyd-ss]

Bob,
Good to see you here!  A lot of talk about carbines and barrel lengths going on these days.  You said you are going to try and tune this carbine for max performance before dropping it to the non-PAL 500fps limit.  Just to make sure I understand, you will be trying to max out the fps with this set-up with the 16"  9mm barrel? That will be very interesting to see.

Regarding your efficiency numbers.   When you quote numbers of 1.2 FPE/cuin, you are referring to 1.2 FPE/standard cuin of air, correct?  Could you show us a calc to show how much air one of those 52FPE shots at 1500 psi would use in cc's.

Thanks for posting!
Lloyd

[rsterne]

HI Lloyd....

Yes, my plan is to see what I can get out of this gun with the 16" barrel before detuning it....

The way I calculate the efficiency is as follows (using the 10 shot string @ 52 FPE)....

Total energy of the string is 10 shots averaging 51.4 FPE which is 514 FPE....

Starting pressure was 2000 psi.... after the 10th shot, there was 400 psi remaining so the pressure used was 1600 psi which is 1600/14.5 = 110 Bar....

The volume of the reservoir is 62cc which is 62/16.4 = 3.78 cubic inches.... Multiply that by the pressure drop (in Bar) and we get 3.78x110 = 416 cu.in. of air used at STP (1 Bar)....

What that means is that by using 416 cu.in. of air we are producing 514 FPE.... If we divide the FPE by the cu.in. air used we get 514/416 = 1.24 FPE/ci of air used at 1 Bar.... Some people like to do the calculation the other way up and using cc.... This works out to 16.4/1.24 = 13.2 Barcc/FPE for those that prefer those units.... I like the former method because a bigger number means that the gun is more efficient....

You can only calculate the amount of air used for one shot if you have an accurate pressure reading between every shot (which I don't).... Generally, a non-regulated PCP is more efficient at the beginning of a shot string when the valve is only open for a very short time and the pressure is very high.... That incidently makes the gun quieter.... At the end of the string, the valve is open longer, there is more noise (report) generated, and the efficiency is lower.... However, if we use the average from above, we will find that for each 51.4 FPE shot, there is (on average) 41.6 cu.in. of air used.... If the average pressure during the shot was 1500 psi (which is 1500/14.5 = 103 Bar) then the volume of high pressure air used during that single shot would be 41.6/103 = 0.404 cu.in. which is 0.404x16.4 = 6.6 cc of air at 1500 psi.... 

I have tested a number of PCPs and have found that the efficiency varies over a wide range.... I have recorded values at high as 1.97 FPE/ci (for a low velocity gun producing lots of shots).... to a low of 0.57 FPE/ci (for a 2260 HPA conversion putting out over 50 FPE in .22 cal).... I'm sure that even higher and lower values are possible....

Bob

[JonnyReb]

 awesome thread guys, a bit beyond me but i'm trying to learn something..   Beautiful gun too! glad you've joined us!  Jeff

[lloyd-ss]

HI Lloyd....

Yes, my plan is to see what I can get out of this gun with the 16" barrel before detuning it....

The way I calculate the efficiency is as follows.......... If the average pressure during the shot was 1500 psi (which is 1500/14.5 = 103 Bar) then the volume of high pressure air used during that single shot would be 41.6/103 = 0.404 cu.in. which is 0.404x16.4 = 6.6 cc of air at 1500 psi.... 

Bob

Hi again Bob,
The last part of what I quoted from you about 6.6cc at 1500 psi to get 41.6 fpe, then it follows that at say 1400 psi the calc works out to 7.07 cc, using slightly more tank volume with each subsequent shot.
The math behind it is very nice and makes good sense.   The wheels are turning.

Thanks,
Lloyd

[rsterne]

That would be correct.... Each subsequent shot would use a bit more volume from the tank.... but of lower pressure air.... The MASS of the air for each shot would be relatively constant.... If the gun was the same efficiency throughout the pressure range (and every shot was the same FPE) then the mass of air used for each shot would be a constant....

In my experience, the pressure drop in a non-regulated PCP is not linear.... Take for example, the Disco... The first five shots, starting from 2000 psi only drop the pressure about 100 psi (20 psi per shot).... By the time you are half way through the shot string, the pressure is down to about 1600 psi, and the pressure drop is about 150-175 psi for 5 shots.... By the time you get to the end of the usuable string, the pressure loss is about 225 psi for 5 shots (over 40 psi per shot).... At the end of the string you are using about twice the volume but at just over half the pressure.... It the efficiency of the Disco was the same from 2000 psi down to 1000 psi you would be using exactly twice the volume at half the pressure (ie the same mass of air).... However, the Disco is less efficient at the end of the string.... so you are using MORE than half the volume at MORE than half the pressure for the last five shots than for the first five shots....

Bob

[rsterne]

I finally got some more pellets so that I could finish off the testing on this Carbine.... I replaced the Disco hammer spring with a 1.75" long one made from 0.040" wire.... This spring in a Disco allows to valve to open with pressures up to 2400 psi.... and it did the same in this 9mm conversion.... With the full size transfer port, and the hammer spring wound in just shy of coil bind I was able to break the 600 fps mark, peaking at 606 fps with the 77.8 gr. EunJin pellets (63.5 FPE).... The shot count at that power level, however, is down to just 9 shots, even allowing a 10% variation (ie a low of 545 fps).... The average velocity over the middle 7 shots of the string is 591 fps (60.6 FPE) with all shots within 94% of the maximum.... Over the whole 9 shots, the pressure dropped from 2300 psi at the start to just 600 psi after shot 9.... and the efficiency worked out to 1.19 FPE/ci....



I then fitted the 0.140" transfer port I had made previously, and things began to get VERY interesting for the non-PAL version.... nice flat shot strings, starting from 2000 psi.... I tried ports from 1/8" to 5/32" and learned a LOT about the effect of strangling the transfer port.... With a 1/8" port, the first shots were the fastest (up at about 2000-2200 psi).... With a 5/32" transfer (and less hammer spring preload) I couldn't use over about 1600 psi, and the curve was getting too short and steep.... I ended up with a transfer port of only 0.136" (#29 drill) which is actually a fraction SMALLER than a stock Crosman port (0.140").... With the hammer spring set about 1.7 turns out from coil bind I could actually get 14 shots within 10% of the maximum, which was 498 fps.... starting from 2200 psi and shooting down to about 700.... I decided that I would sooner have a shorter but flatter string.... so I settled on filling to 2000 psi and shooting down to about 800.... That gave me 11 shots averaging 489 fps (41.4 FPE) with a high of 498 and a low of 473.... only a 5% variation across the string.... The efficiency was an extremely high 1.53 FPE/ci.... To give you an idea of how efficient Big Bores are, a stock Disco can barely get to 1.30 FPE/ci....   

To say I'm pleased with the results would be an understatement.... I now have a non-PAL 9mm Carbine that shoots over 41 FPE for 11 shots per fill....   

Bob

[lloyd-ss]

Bob,
Did I understand correctly that you are doing this with a tiny 62cc reservoir?!
Thanks,
Lloyd

[trekkerx]

I have been interested in building a 9mm Disco myself but would like it to be a full length rifle.I was thinking about a Lothar Walther barrel at 24 inch or longer.I don`t have a lathe or mill to build the breech so I have yet to get started on this project.I sit here with calculator, pen & paper & try to figure out all these #`s you speak of I`m now wondering if I can build one that would be efficient on air or not.I have built a couple .22 Disco`s & enjoyed building them but in terms of efficiency I don`t think my builds are anything to brag about.If you guys keep on explaining the mathmatics on air usage maybe I`ll create a great rifle in 9mm out of Disco parts.Now who can build a breech in 9mm?

[rsterne]

Yes, the air reservoir is a 2260 tube.... about 62cc.... less than half that of a Disco.... 

Bob

[r1derbike]

How loud is the report, from it? Compared to a stock .22 Disco?

Charles

[Rescue35]

Wow, starting to look like my ~350cc resivoir might be overkill. Good thing since the next one I build I was already planning to use a smaller tube.

trekkerz: machining the breech isn't that difficult. I Used square bar aluminum like rsterne did, great minds think alike.

[rsterne]

It's pretty loud!.... after all, about 80% more FPE than a stock disco and only 2/3rds the barrel length.... 

I can get away with the small reservoir because I restricted the velocity to under 500 fps to meet the Canadian laws for not requiring registration.... I have my PAL but it's nice not to have to register every build....   

Bob

[rsterne]

I did some quick calculations on how a 24" barrel and a full length Disco tube (135 cc) would affect this gun.... With no other change, the velocity would increase about 30 fps due to the longer barrel, and the efficiency would obviously increase.... Backing the hammer down (or restricting the transfer port more) to stay under 500 fps would gain a bit more efficiency.... My calculations show about 25 to 30 shots per fill with the same shot string characteristics.... ie ~490 fps (41 FPE) in 9mm.... That's more shots than a stock Disco at about 80% more power....  

Bob

[rsterne]

I decided to see how the stronger hammer spring I purchased for my Hayabusa would work in this gun today.... I put back in the full size (0.218") transfer port and changed out the springs and set the preload to just shy of coil bind.... I filled the gun to 2000 psi and the first shot ripped across the Chrony at over 650 fps.... Talk about LOUD!!!....

After playing with the fill and refill pressures I came up with the following 5 shot string.... I filled to 2400 psi and in just 5 shots the pressure dropped to 1200.... 77.8 gr. EunJin pellets as before, and still with the 16.5" barrel....

639
655
664 - 76.2 FPE @ ~1900 psi
658
642

The average velocity is 655 fps (74.1 FPE) for a total energy of 371 FPE.... The air used is 310 CI so that works out to 1.19 FPE/CI (13.8 Barcc/FPE).... That is nearly the same efficiency as a stock Disco at over 3 times the power....  I did the calculations for a 24" barrel and it predicts 710 fps (87 FPE) for a peak and 700 fps average (85 FPE) over the 5 shot string.... with the efficiency rising to 1.36 FPE/CI.... Using a Disco tube (over twice the volume), that should give a 10 shot string between 2400 - 1200 psi within just a 4% velocity window.... That would make quite a usable hunting rifle.... basically 4 times the FPE of a stock Disco....  

This platform continues to amaze me.... I wouldn't be surprised if it were possible to make over 100 FPE with a 9mm Disco by using heavier bullets....  

Ohhhhhhhhhhhhhh Lloyd.... Here is a perfect challenge for your dual tube Disco....  

Bob

[lloyd-ss]

Scary Bob!  You are getting that 74fpe with basically the same mods we have been talking about, right? You are still using the stock hammer, correct?  DO you have any idea what fpe it would get if you put a .25 barrel back on it?  I am thinking the deciding factors on those big fpe numbers are the bigger bore with the proportionately light pellets, and keeping the velocity in the more efficient lower fps range.
Excellent results, and it is begging for that second air tube, isn't it!
Lloyd
 

[rsterne]

Stock hammer, 1/4" valve throat, 3/16" exhaust port (angled 20* and streamlined), 7/32" transfer port (only 'cause I could cheat and use a piece of brass K&S modelling tube), IIRC the barrel port is only 3/16".... Disco valve streamlined on the inlet but not bored out, and no gauge port to hamper flow into the valve.... 1.75" x 0.045" wire hammer spring.... Actually not that highly modded, pretty much the kind of thing anyone could do.... OK except the riser breech and bolt....  

I never had a .25 barrel on this gun, but I do have a 24" .357 barrel coming, and I have a Disco tube.... I also have a 24" .25 cal barrel coming so I will be trying that on the Disco tube at some point in the future as well.... Too many projects....  

The 78 gr. EunJin is the same Sectional Density as a 29 gr. pellet in .22 cal, and a 38 gr. in .25 cal.... so it's not exactly a "lightweight" pellet.... I think the limiting factor is the size of the valve throat and ports (starting with a Disco valve) on a 9mm.... I actually think that in .25 cal, the Disco valve can be made to flow all that is necessary, especially since the most accurate pellets seem to be the JSB King 25 gr. and the H&N Baracuda 31 gr.... Remember my "rule of thumb" velocity of 950 fps.... where the energy is twice the pellet weight in grains.... So in .22 cal, we "only" need 50-60 FPE.... unless somebody comes up with some really heavy pellets that have superb accuracy....

However, to get the 78 gr. EunJins to 950 fps requires 156 FPE.... I have my doubts we can push a Disco valve that far.... 

Bob