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All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: rsterne on December 24, 2011, 05:12:46 PM

Title: 9mm and .308 cal Disco Build
Post by: rsterne on December 24, 2011, 05:12:46 PM
I got some time over the past few days to start my Big Bore Disco project.... I started with a collection of Disco parts, some new, some used, and I made a copy of the Disco gauge mounting block (complete with the stock 1/8" through hole) and built what amounts to a stock Disco main tube assembly, including a stock fill fitting, valve, hammer with cocking pin, hammer spring, end cap, and trigger assembly.... The trigger assembly and the hammer were polished, and the trigger spring was changed for a lighter one and the "3-screw" mod was done, so I have a nice 2-stage adjustable trigger.... I didn't bend the sear spring to reduce the second stage pull weight, so the total trigger pull is just over 2 lbs....

I recently received some new hammer forged barrels from Stan at AERO-TEC, including a 24" long .357 cal with a 6-groove 1 in 16" twist and a 24" long .308 cal with a 4 groove 1 in 20" twist.... Both barrels are 1/2" OD, and I also received the prototype of a Big Bore Riser breech I designed.... The breech is made from a 3/4" x 1" billet of 6061-T6 aluminum and anodized.... It has a 3" deep socket for the barrel with two 8-32 set screws to locate it.... The front of the breech has a short stub designed to mount a 3/4" OD x 0.035" wall aluminum tubing shroud, located by two 6-32 screws.... There is plenty of clearance between the main tube and the barrel to allow for the shroud.... The idea is to make the two barrels interchangeable, using the same breech and bolt assembly.... I machined identical barrel ports in them with a 7/32" through hole and a shallow 1/4" counterbore for the transfer port to sit in.... The transfer port is made from a short piece of 1/4" brass rod with a shoulder on the bottom for the stock Crosman TP seal.... although a poly port can also be used.... The barrels both have small flats for the setscrews, and the chambers were cut using a tapered reamer so that the portion between the barrel port and the end is slightly over 3/8" to clear the nose of the bolt.... In the case of the .357 barrel, that meant the rifling started about the middle of the barrel port and was full depth about 1/4" forward of the port.... On the .308 barrel, the rifling starts ahead of the transfer port, and in fact you can push a 120 gr. bullet far enough that the base of the bullet is about level with the back of the barrel port before it touches the rifling....

(http://i378.photobucket.com/albums/oo221/rsterne/9mm%20PCP%20Carbine/9mm30calDisco.jpg)

The rear mounting screw for the breech is 8-32, and the head protrudes into the bolt passage about half way.... The front mounting point for the breech is still under the bolt in the loading trough, but it is increased to 6-32, and the front hole in the Disco tube was drilled and tapped to match.... A small recess was ground in the top rear of the Disco valve to miss that screw.... One thing I noticed about this Disco valve was that it didn't have the usual O-ring seal for the valve stem.... I don't know if Crosman forgot it, or if they are cheaping out and leaving it off on purpose.... The weird thing is that there is no machining to even allow the installation of the O-ring, the back of the valve is just flat.... I have purchased one Disco valve since this one and it had the O-ring again.... so I don't know which is current production....

The breech seal is achieved by a 1/2" OD x 3/8" ID O-ring held captive between the end of the barrel and the breech.... The bolt, which has a probe to push the pellets past the transfer port, has a 3/8" nose that seals in that O-ring.... Since the O-ring is pinched just slightly between the barrel and breech, the ID was a bit under 3/8" and it was necessary to polish down the nose of the bolt slightly for the proper fit.... A 9mm pellet slides through the O-ring without friction, however.... The rear section of the bolt is 9/16" diameter.... This allows the bolt to catch the stock cocking pin for the hammer.... but in order to do that, it needs a groove machined in it, wider at the rear to miss the head of the rear mounting bolt....  Once the bolt is rotated to the locked (firing) position, there is an addditional (offset) groove required for the cocking pin to travel in without touching the bolt.... That groove is the one closest to the bolt handle.... I know it looks complicated, but the machining can be simplified now that I have made this prototype....

(http://i378.photobucket.com/albums/oo221/rsterne/9mm%20PCP%20Carbine/BreechParts.jpg)

The bolt is designed so that when drawn to the rear, the front mounting hole is exposed, and when forward, the rear screw is just behind the back of the bolt.... The breech assembly can therefore be removed without removing the bolt from the breech first.... The slot allows for a large travel, and the loading port is long enough for at least a 120 gr. (maybe even a 150 gr.) .308 bullet.... The bolt handle is made from a 2" long 10-32 SHCS with the threads shortened.... The hole in the bolt is 1/2" deep (ie blind) and the screw tightens against the bottom of the hole.... The knob is from a piece of 5/8" diameter brass, 7/8" long.... It is drilled 5/16" to a depth of 0.70" and then tapped 3/8-16 NC for a 1/2" long stainless SHSS.... When the set-screw is tightened, it pinches the head of the bolt handle, making the handle assembly rigid.... The handle is installed using the 3/16" hex in the end.... Also shown is a barrel band which matches the breech in height.... It has an 8-32 set-screw in the lower portion, but has an O-ring in the upper portion to "semi-float" the barrel.... I have another band which is designed to be used with a 3/4" OD shroud, located by a set screw in both openings....

Now to put this thing together and take a few shots.... I expect the velocity to be well under 500 fps with all the stock Disco components.... I plan to test it on CO2 as well as on HPA, so stay tuned....

Bob
Title: Re: 9mm and .308 cal Disco Build
Post by: Butcher45 on December 24, 2011, 05:30:17 PM
I've always liked the idea of a .308/.32 Disco with a twist rate suited for lightweight slugs.  Shrouded.  What a fun walk-around sqwerl gun that would be!

I'll be watching this build with enthusiasm.
Title: Re: 9mm and .308 cal Disco Build
Post by: Gary on December 24, 2011, 06:27:06 PM
This interests me. Have you done any mods to the valve? I wouldn't think a stock valve would have enough power to push a bullet out the barrel but what do I know.
Gary
Title: Re: 9mm and .308 cal Disco Build
Post by: airpuffhunter on December 24, 2011, 06:43:54 PM
come on Bob, those disco conversion are always interesting.
I hope it goes well, and perform better than spected
Title: Re: 9mm and .308 cal Disco Build
Post by: Rescue35 on December 24, 2011, 07:12:45 PM
so stay tuned....

Bob

Like you had to tell us to do that.

Looking good Bob, keep it up.
Title: Re: 9mm and .308 cal Disco Build
Post by: rsterne on December 24, 2011, 07:54:38 PM
Just a quick report.... The Disco valve does leak out the back around the stem when fired, so I replaced it with another which does have the O-ring.... I also have some blow-by past the O-ring that seals the bolt, I'm going to have to chase that down.... As I figured, the small hole in the gauge block is starving the valve for air and the velocities are down quite a bit compared to the Carbine I did.... I don't think the lack of velocity is due to the enlarged exhaust ports in the valve in the Carbine because I got 498 fps with a stock diameter (0.140") transfer port.... In any case, here are the preliminary results, with two different hammer springs and on CO2.... 77.8 gr. EunJin pellets....

Stock Disco valve and ports, stock gauge block (1/8" hole)

Pressure .... Disco Spr.... 0.040" Spr.

2000 psi ...... 290 fps ...... 355 fps
1500 psi ...... 350 fps ...... 390 fps
1000 psi ...... 350 fps ...... 380 fps
800 psi ....... 340 fps ...... 340 fps

CO2 (70*F) ... 315 fps ...... N/T

The next step will be to drill out the through hole in the gauge block to at least 1/4" so that the average pressure at the inlet of the valve during the shot cycle will be higher.... With the tiny 1/8" hole (which is stock for a Disco) I'm effectively trying to get 40+ FPE of output from only about 5cc of HPA.... and as you can see, it's just not working.... I know the potential is there because I saw over 70 FPE from the Carbine during testing.... Now to pull it apart yet again....

Bob
Title: Re: 9mm and .308 cal Disco Build
Post by: rsterne on December 26, 2011, 10:28:11 PM
I drilled out the gauge block today to a 1/4" through hole, and as expected the velocity increased.... However, it was still under 500 fps....

Stock Disco valve, gauge block drilled to 1/4", 0.040" wire spring @ stock preload

2000 psi .... 390 fps
1500 psi .... 450 fps
1000 psi .... 390 fps

The gun is getting a lot of shots, I'm guessing well over 20 between those pressure points.... It would appear that the ports in the stock valve are simply not big enough and/or smooth enough to flow enough air to get over 35 FPE.... Since my Carbine, with a shorter barrel, did that at similar pressures (the valve was ported out) it was time to pull the gun apart again and hog out the valve.... I milled out the exhaust port to 3/16" on a 20 degree angle, drilled the valve throat to 1/4", and streamlined it with my Dremel.... Then I bored out the valve to 0.62" ID (mostly to allow more flow around the head of the poppet), leaving only 2 threads, and shortened the front end to match.... I then tapered the inlet portion and countersunk both ends of the gauge port to streamline the flow.... There is still more that can be done by making a new valve front end with larger passages to eliminate the gauge block, and thinning the valve stem, but I'm trying to learn as much as possible by doing things in a progressive manner.... I also fixed the air leak around the nose of the bolt by simply lubricating the O-ring so that it could move a bit.... Sometimes the fix is almost too simple.... This time I saw quite a large jump in velocity.... Still using the 77.8 gr. EunJins....

Bored out valve and ports, streamlined gauge block and valve inlet, 0.040" wire spring @ stock preload

2000 psi .... 460 fps
1600 psi .... 525 fps
1400 psi .... 545 fps
1200 psi .... 540 fps
1000 psi .... 490 fps

I decided to try a heavier hammer spring, just shy of coil bind and got a few of shots right around 2000 psi of 660 to 680 fps (80 FPE).... so the potential is definitely shaping up.... I plan on doing the remaining valve mods next and eliminating the restriction of the gauge block....

Bob
Title: Re: 9mm and .308 cal Disco Build
Post by: rsterne on December 28, 2011, 10:23:53 PM
I made a new valve front end today to incorporate the gauge block and increase the diameter of the through hole.... I moved the O-rings to in front of the gauge hole so that I wont have to deal with shredding O-rings as they pass the hole in the tube for the gauge.... Even though it's deburred, it's a PITA to keep the O-rings stuffed deep enough as they slide past.... so now I don't have to worry about it.... While I was at it, I shortened the threads by 0.20" to help open up the flow inside of the valve....

(http://i378.photobucket.com/albums/oo221/rsterne/9mm%20PCP%20Carbine/ValvesSide.jpg)

One of the choke points was the hole in the spring seat in the valve front end, and there is little point in drilling it out because the spring is just over 1/4" ID.... Sooooooooo, I tapered the spring.... I used progressively larger drill bits, inserted the shank end into the spring, and tapped the spring with a hammer on an anvil to increase the diameter.... I ended up with a spring that is stock at one end to fit the valve poppet, and 3/8" OD x 19/64" ID at the other end.... I stress-relieved the spring afterwards by heating it at 500*F for 1/2 hour.... The larger spring ID meant I could drill the valve front end out to 19/64 which increased the area for airflow into the valve by 41%.... Compared to the restricted flow through a stock gauge block (1/8") it's an increase of over 5 times.... The valve now has 2.5 times the area on the inlet end than on the exhaust end.... so the entire reservoir can keep the pressure up in the valve during the shot cycle....   

(http://i378.photobucket.com/albums/oo221/rsterne/9mm%20PCP%20Carbine/ValvesEnd.jpg)

Other mods include drilling the valve throat out to 1/4" (from 7/32") and thinning down the valve stem from 5/32" (0.157") to 0.115" The exhaust port in the valve has been opened up to just over 3/16" (from under 5/32") and angled 20 degrees and streamlined.... The new ports are 70% larger than stock and a much more direct path.... I also bored out the inside of the valve to 5/8" (from about 1/2"), not so much to increase volume as to unshroud the head of the valve poppet and ease the passage of the air around it.... and shortened the threaded area as well.... The threads are in compression, all they really do is hold the two halves of the valve together to seal the O-ring between the halves.... so strength is not really an issue there.... The tightest restriction in the whole system is the exhaust portion of the valve, everything else has greater flow areas.... I also shortened the valve stem by 0.090".... It now barely protrudes from the end of the valve when the spring hits coil bind (which limits the travel).... and the increased hammer travel will allow the hammer to have increased momentum when it strikes the valve....

All in all, I'm pleased with the progress and look forward to reassemblling the gun with the new valve parts.... Once those trials are completed I will likely work on a new hammer....

Bob
Title: Re: 9mm and .308 cal Disco Build
Post by: Rescue35 on December 29, 2011, 12:09:48 AM
Not only do you do great machine work and understand the fluid mechanics that take place inside an air gun but you do a great job of documenting and explaining all of it. Thanks Bob and I'm looking forward to your next installment on this project.
Title: Re: 9mm and .308 cal Disco Build
Post by: rsterne on December 29, 2011, 01:18:49 AM
Thanks Rescue35.... I've only been doing this for a bit over 4 years and when I started I was sooooooooooo frustrated at the lack of good information and documentation I decided to do what I could to change that.... so as I learn, I record and share as much as possible.... Nice to know it's appreciated....  ;D

Bob
Title: Re: 9mm and .308 cal Disco Build
Post by: RWSshooter on December 29, 2011, 01:35:59 AM
You've got my attention!  Great reading!  The workmanship looks very clean.  Thanks for sharing your experience!
Title: Over 90 FPE Today !
Post by: rsterne on December 29, 2011, 10:50:18 PM
I got a chance to assemble the rifle today using the new valve.... and I'm very pleased with the results.... When I first tried the hammer spring setup I was using previously (1.75" long x 0.040" wire with the stock rear plug) I only got ~300 fps.... It only took me a minute to understand that since I shortened the valve stem the preload on the spring was quite a bit less.... I replaced the spring with a 2.00" long one (also 0.040" wire) and the velocity jumped to over 600 fps (at only 1000 psi ! ).... Now we're talking!.... I then took off the fixed end plug and installed one of my RVAs which has the added advantage that I can change out hammer springs without taking off the stock and the end plug.... I tried three different 2.00" long springs, the other two had wire sizes of 0.042" and 0.045".... I adjusted all three just shy of coil bind (ie maximum safe preload) and got the following results.... Only shots over 95% of the maximum velocity are shown....

(http://i378.photobucket.com/albums/oo221/rsterne/9mm%20PCP%20Carbine/9mmDiscoSprings.jpg)

The velocity with the 0.040" wire spring peaked at about 1400 psi at 666 fps (77 FPE) and I got 10 shots averaging 649 fps between 1800 psi and 900.... With the 0.042" spring the peak velocity was 713 fps (88 FPE) at 1600 psi and starting from 2000 psi I got 8 shots averaging 699 fps ending at 1200.... With the 0.045" wire spring I got 7 shots averaging 716 fps starting from 2200 psi and ending at 1200, with the peak velocity being 733 fps (93 FPE) at 1750 psi.... The efficiencies were 1.39 FPE/CI, 1.22 FPE/CI, and 1.12 FPE/CI respectively, so I'm quite pleased with that.... I'm pretty sure that this gun can be tuned for a few of shots at 100 FPE with a 2400 psi fill pressure and more hammer strike.... With the current hammer setup, however, I'm pretty well tapped out I think.... At least I got to put the stock on it today.... here's what she looks like....

(http://i378.photobucket.com/albums/oo221/rsterne/9mm%20PCP%20Carbine/BigBoreDiscovery.jpg)

The next step is to make a cylindrical hammer (ie no waist) and I plan to encorporate an external cocking knob with integral spring guide.... This will increase the hammer weight (and momentum) and while I'm at it, I'll probably set it up for bigger hammer springs as well.... it will depend on what I can find for springs.... While that will allow me to explore the limits of the rifle.... when you consider the small air reservoir and the fact that I already can tune the gun for maximum power with a 2000 psi fill.... they may not be very practical.... Those parts will likely be experimental only and I will probably return to the 0.045" spring and detune the gun slightly below the current maximum.... likely by installing a slightly smaller transfer port to flatten the curve and (hopefully) increase the shot count....

Time will tell.... In the meantime, I'm thoroughly enjoying this build and I'm learning a TON....  ;D

Bob
Title: Re: 9mm and .308 cal Disco Build
Post by: Rescue35 on December 30, 2011, 10:34:45 AM
Wow Bob,

When you mentioned shortening the valve stem I thought about the hammer also. Have you thought about turning a longer hammer (proportional to the length of stem removed) and returning to the 1.75" long spring. That would add a fair amount of mass. Although it really is mainly academic at this point. You are getting some great numbers there. This thread will be getting printed out and added to the notebook.
Title: Re: 9mm and .308 cal Disco Build
Post by: rsterne on December 30, 2011, 01:18:15 PM
Hammer travel is very important because the hammer has more room to accelerate to pick up velocity and hence energy and momentum.... If you keep the travel the same and change the weight of the hammer, you increase the momentum but NOT the energy.... If you keep the weight the same and increase the travel, you increase BOTH energy and momentum.... assuming you have enough spring preload to still be accelerating the hammer when it reaches the valve.... That was my problem - when I shortened the valve stem, the short spring (at low preload) ran out of steam before impact.... Increasing hammer travel works great, but you inevitably have to play with the spring.... I never tried the 1.75" spring with increased preload because I needed more power anyway and went to a heavier wire.... I do, however, have a 0.045" wire spring that is 1.75" long and it has a higher spring rate than the 2.00" spring.... I will try it (before changing the hammer), but it may not have enough travel to deal with the increased hammer travel....

There could have been another factor involved as well.... When I opened up the porting on the inlet side of the valve I increased the AVERAGE pressure during the shot cycle.... because air can rush into the valve from the reservoir easier than it can get out.... This increased pressure was likely slamming the valve shut against the now weaker spring (less preload).... and reducing the velocity in much the same way as it falls off when the valve partially "locks" in the first half of a PCP cycle....

Regarding changing the hammer length as you suggest.... it would no longer be able to catch the sear.... Plus, it would eliminate the extra travel I gained by shortening the stem, which was the whole point.... I will be making a cylindrical hammer (with cocking pin hole) that I can try by itself.... but the back end will be threaded for a cocking rod / spring guide which can be installed also.... That rod will run through the adjusting screw in the RVA and have a cocking knob on the end of it for external cocking.... The entire assembly will be significantly heavier, which will add to hammer momentum.... Lots of things to try yet....  ;D

Bob
Title: Re: 9mm and .308 cal Disco Build
Post by: Rescue35 on December 30, 2011, 02:13:28 PM
I didn't think about the sear engagement. Duh.

Since f=m X A my line of thinking, since you had more room meant you could add mass up front. I like your idea on the external cocking, I think you had done this with one of your other guns. That will gain you the benefits of both. More mass and acceleration.
Title: Re: 9mm and .308 cal Disco Build
Post by: lloyd-ss on December 30, 2011, 05:30:35 PM
Bob,
Nice project and very nice of you to share such great documentation with everyone.
Lloyd
Title: Re: 9mm and .308 cal Disco Build
Post by: echochap on January 03, 2012, 09:39:27 AM
Nice gun and the best write up I've seen on a big bore build. That's a good shot string in the 700s, seems like a compact little mid range gun for medium size quarry. Again, excellent job building and communicating!
Title: Re: 9mm and .308 cal Disco Build
Post by: rsterne on January 19, 2012, 05:17:13 PM
I spent a bit of time in the shop today.... it sure felt good to be back out there.... I tried a 1.75" long 0.045" wire hammer spring, and also the same length from 0.051" wire.... The lighter one wasn't as good as the 2.00" long one I tried previously, and the heavier one was only marginally better, peaking at 736 fps.... but it was VERY difficult to cock.... So far (with the stock hammer) the best spring is the 2.00" long x 0.045" wire one....

I had a few 132 gr. cast bullets that had been sized to 0.357", and tried them out.... They were VERY hard to load, I actually had to tap the bolt closed.... They only hit 540 fps (85.5 FPE), probably because of the tight fit.... I will have to get a smaller sizing die to use with this barrel if I want to try anything other than the EunJins....

The next project is to make a new hammer with rear cocking knob and the ability to accept longer/stiffer springs....

Bob
Title: Re: 9mm and .308 cal Disco Build
Post by: ronbeaux on January 19, 2012, 09:00:43 PM
I soo look forward to your posts and knowledge. I am committed to advancing in the machine techniques. Thank you!!
Title: Re: 9mm and .308 cal Disco Build
Post by: lloyd-ss on January 20, 2012, 02:44:10 AM
......... I had a few 132 gr. cast bullets that had been sized to 0.357", and tried them out.... They were VERY hard to load, I actually had to tap the bolt closed.... They only hit 540 fps (85.5 FPE), probably because of the tight fit.... I will have to get a smaller sizing die to use with this barrel if I want to try anything other than the EunJins....

....
Bob
Bob, I have some"real" sizing dies and then I have some home made ones that are basically blocks of steel bored out to the correct size.  They don't do a very pretty job, but they work well enough for getting a correct fit for velocity testing.  I just put the bored block on the drill press and use a drill blank in the chuck to push the bullet thru the die.
Title: Re: 9mm and .308 cal Disco Build
Post by: rsterne on January 20, 2012, 12:59:15 PM
It might be interesting to try several sizes and find the one that produces the highest velocity.... I wonder if I can make a universal holder for my Lee press and a series of inserts of different sizes?....

Bob
Title: Re: 9mm and .308 cal Disco Build
Post by: lloyd-ss on January 20, 2012, 02:04:22 PM
It might be interesting to try several sizes and find the one that produces the highest velocity.... I wonder if I can make a universal holder for my Lee press and a series of inserts of different sizes?....

Bob
Ha, Ha.  Bob, I think that is a rhetorical question.  Have you started making them yet?

This place seems to have all the sizes in stock with the best prices.  Anybody found better??
http://www.titanreloading.com/lube-sizing-kit (http://www.titanreloading.com/lube-sizing-kit)

I have tried opening one of these sizing dies up with abrasive paper on a drill press and had to work and work to get a few tenths.  They seem to be hardened, which makes sense.  Have you tried it or had any success?
Lloyd

Title: Re: 9mm and .308 cal Disco Build
Post by: melloroadman on January 20, 2012, 10:47:48 PM
Lloyd The last one I opened up .001 in about 1 minute after set up . 180 grit paper rolled up tightly around a small diameter drill bit . Paper was at least 1/4 inch thick after roll . OD was so large I could hardly insert it into the die . Had to apply oil to paper to get it into the die. Chucked it into a 1/2 inch drill that was not plugged in . Plugged drill in , pulled trigger , released trigger , un plugged drill , removed rolled paper from die , tried the die out and I had gone from .452 to .453 . That was after I had tried fine grit paper by hand for 15 minutes . Yes they are hardened and some are harder than others . 180 did a fast job on this one .Marvin
Title: Re: 9mm and .308 cal Disco Build
Post by: lloyd-ss on January 21, 2012, 12:30:06 AM
Marvin, OK, I will give it another try.  Maybe I was being too cautious and need to try some more aggressive paper like the 180 you used.  I'll give it another shot.
Thanks,
Lloyd
Title: Re: 9mm and .308 cal Disco Build
Post by: rsterne on January 22, 2012, 09:03:47 PM
I got some time in the shop today, so I made a rear-cocking hammer for this gun.... The hammer is cylindrical (ie no waist) and the same length as a stock Disco hammer.... It is drilled deeper for the longer hammer spring, and also drilled and tapped 10-32 for the spring guide / cocking rod.... The hammer spring is 2.5" long x 0.360" diam, and I have 2 wire sizes, the smaller being virtually identical in spring rate to the hammer spring in a QB78.... I installed a stock cocking pin so that the hammer can be used in self-cocking mode if desired....

(http://i378.photobucket.com/albums/oo221/rsterne/9mm%20PCP%20Carbine/RearCockingHammer.jpg)

The combined weight of the hammer, cocking rod, and knob is 90 gr.... which is 50% heavier than a stock Disco hammer.... The hammer alone is 59 gr.... and the total weight can be adjusted by changing the weight of the cocking knob.... The barrel of the adjuster is made from aluminum, and the screw is made from a 3/8"-16 stainless SHSS, shortened, drilled, and two flats ground on it for an adjusting wrench....

(http://i378.photobucket.com/albums/oo221/rsterne/9mm%20PCP%20Carbine/HammerParts.jpg)

The range of adjustment allows me to set the preload from zero, uncocked.... all the way to coil bind, cocked.... The knob is easy to grasp and the hammer is not difficult to cock even at full preload.... although with the heavier spring I'm sure it would be a bear to pull back.... I doubt that spring (which is twice as stiff) will be necessary anyway.... There is already a mark on the hammer from the sear, so before I install it I am going to harden it.... It is 1045 steel, so will harden easily by heating to a dull cherry and quenching in water.... Once it is hardened I will be conducting more tests to see if I can get this gun over 100 FPE.... if only for a few shots....

Bob
Title: Re: 9mm and .308 cal Disco Build
Post by: lloyd-ss on January 23, 2012, 01:07:37 AM
Nice looking work Bob. So you are going after the big fpe on this one.  I'll be watching.
That last closed coil on the spring looks very precisely done.  How did you manage that?  I always struggle to make them look like a "factory" spring.
Thanks,
Lloyd
Title: Re: 9mm and .308 cal Disco Build
Post by: rsterne on January 23, 2012, 01:38:47 AM
It's a factory spring.... I buy most of my springs from Trakar Spring in Ontario, Canada....

Bob
Title: Well Over 100 FPE
Post by: rsterne on January 23, 2012, 10:10:30 PM
I got the hammer hardened and installed today.... It's very easy to cock even at maximum preload on the hammer spring (near coil bind).... The first few shots showed me that it's PLENTY of hammer hit to dump 2400 psi (or more), as the first shot over the Chrony was 835 fps with the 77.8 gr, EunJins (120 FPE).... I progressively backed down the preload until the second shot was the fastest (4 turns out) and then started shooting shot strings all the way down to minimum preload.... At 6 turns out from coil bind, the gun performed virtually the same as it did with the stock hammer running the 0.045" x 2.00" spring at maximum preload.... so between the heavier hammer and the longer, stronger spring I have gained a lot.... At 8 turns out (minimum) the velocity peaked at about 600 fps and at 7 turns out at about 700.... The other results are in the graph below....

(http://i378.photobucket.com/albums/oo221/rsterne/9mm%20PCP%20Carbine/9mmDiscoRearCocking.jpg)

The pressure graph is only approximate.... It's slightly different for each setting, but it gives you an idea of what is going on.... At maximum, the first shot was 835 fps and the second shot 809.... but the efficiency was only 0.71 FPE/CI.... At 4 turns out, I got a 3 shot string that peaked at 110 FPE (798 fps) and the efficiency jumped to 0.90 FPE/CI.... From there to 6 turns out, the gun was very sensitive to adjustment, so I shot strings every half-turn.... At 4.5 turns out I got 5 shots averaging 103 FPE (771 fps) and 0.93 FPE/CI.... At 5 turns out it was 6 shots averaging 753 fps (98 FPE) at 0.97 FPE/CI.... 5.5 turns out, the velocity averaged 734 fps for 7 shots, producing 93 FPE at an efficiency of 1.03 FPE/CI.... Finally, at 6 turns out, I got an 8 shot string averaging 715 fps (88 FPE) and the efficiency was 1.07 FPE/CI.... The last string started at 2200 psi, all the others started at 2400....

I'm very pleased with the results.... Basically I can choose anywhere from 3 shots at 110 FPE down to 8 shots at 90 FPE or anything in between.... I'm leaning towards the 5 shot string between 100-106 FPE as the best balance between power and efficiency.... One other thing.... I slipped an O-ring over the cocking shaft, so that I can measure directly the amount the valve opens.... idea compliments of Lloyd.... At 4.5 turns out and 2400 psi (shot #1) the valve opens about 0.060", increasing to 0.090" at 2000 psi (shot #3) and about 0.120" at 1600 (shot #5).... It's interesting to see the "balancing act" of hammer hit vs. pressure in action....  This hammer arrangement answered a lot of questions for me.... It certainly removes the inability of the Disco to dump large quantities of air.... and since I was having the same problem with my Hayabusa (insufficient hammer strike) I now know what I need to do to see some much bigger numbers from that gun as well....   ;D

Bob
Title: Re: 9mm and .308 cal Disco Build
Post by: lloyd-ss on January 24, 2012, 08:09:44 AM
Bob,
Very nice work.  I haven't gotten that many FPE out of the Disco yet, so I am glad to know it can be done.  Amazing the range that little valve can be coaxed into performing through.  I was looking back thru the posts in this thread and it looks like you are using the stock heavy valve return spring?  I tried going very light on that spring once and got pretty severe hammer bounce, but there is some balancing that can be done there, too.  The heavy hammer took care of that spring preload. 
Very respectable performance.  Now maybe you need some more air reservoir?
Lloyd
Title: Re: 9mm and .308 cal Disco Build
Post by: rsterne on January 24, 2012, 11:49:37 AM
Gee, you think?.... I guess I know what to use a dual tube setup on, anyway.... :o

Yeah, I'm pretty happy about the performance.... It makes me wonder what could be achieved by upping the valve screws to 10-32 and using 3000 psi.... now that I have a hammer that can open the valve OK at 2400+ psi....  :-\

Bob
Title: Re: 9mm and .308 cal Disco Build
Post by: airpuffhunter on January 24, 2012, 02:43:48 PM
Bob, I have just reread all the tread, this is something really good to try.
what would happen if you increase the valve volume?
Title: Re: 9mm and .308 cal Disco Build
Post by: rsterne on January 24, 2012, 06:40:21 PM
I actually did increase the valve volume, but more importantly, I opened up the entrance to the valve so that air can get in easier than it can get out.... Effectively, that makes the entire air reservoir the valve.... In a stock Disco, the gauge block only has a 1/8" hole through it and severely limits the flow into the valve.... Earlier in this thread I showed the extended valve front end I made to replace that....

Bob
Title: Re: 9mm and .308 cal Disco Build
Post by: airpuffhunter on January 24, 2012, 11:03:51 PM
OK thank you
Title: Re: 9mm and .308 cal Disco Build
Post by: rsterne on January 26, 2012, 05:21:47 PM
I played around with the gun a bit more the last couple of days.... I installed a full length shroud, and although it looks nice it had little to no effect on the noise.... It projects 3" beyond the barrel and is sealed to the barrel 5" behind the muzzle....

(http://i378.photobucket.com/albums/oo221/rsterne/9mm%20PCP%20Carbine/9mmwithShroud.jpg)

I also tried a 2500 psi fill and 4 turns out on the adjuster with excellent results, using the 77.8 gr. EunJins....

758 fps
782
793 - 109 FPE
782
758

It is a completely symmetrical shot string, with a 35 fps spread (4.5%).... The ending pressure was 1500 psi, average energy 104 FPE, and the efficiency was 0.94 FPE/CI.... In addition I sorted some 0.36" buckshot from Ballistics Products into two groups.... About a third of them were under 0.354" and loaded OK, the rest were over 0.357" and were too tight.... I shot a 5 shot string with the smaller size, starting from 2500 psi, and got an average velocity of 812 fps (98.7 FPE).... That string used 1100 psi and the efficiency was only 0.81 FPE/CI.... The pellets are MUCH more efficient than the roundball....

I'm going to have to get a smaller sizing die for my cast slugs, both for this barrel and the .308.... I'm a bit surprised at that, considering the dimensions that the barrels are supposed to be.... This is where my inexperience with big bores is showing.... I don't know how the gun is supposed to be chambered for cast bullets.... Should they load past the transfer port with virtually no effort?.... Should the leade past that point be a shallow taper or a fast one - or no taper at all?....

Bob
Title: 132 FPE With a Disco !
Post by: rsterne on January 26, 2012, 10:07:52 PM
After a discussion on the strength of the valve securing screws in the Disco, I decided it was time to upgrade the screws from 8-32 of unknown strength to 10-32 high-tensile screws.... For the details on how I accomplished that, please refer to this thread....

http://www.gatewaytoairguns.org/GTA/index.php/topic,25091.20.html (http://www.gatewaytoairguns.org/GTA/index.php/topic,25091.20.html)

Now that I don't have to worry about the strength of the valve screws, and given that the strength of the other items in a Disco calculate out with a high enough safety margin to satisfy me.... I now have the ability to try a 3000 psi fill....

NOTE:  If you haven't upgraded the valve screws don't even consider doing this, it would be unsafe.... I'm confident that the strength of the rest of the Disco is OK, but I am NOT recommending that you follow suit.... HPA is dangerous, and using more than 2000 psi in a Disco is done at your own risk....

After installing the valve with the new screws and pressurizing the gun and testing for leaks.... I cranked the hammer spring to the maximum preload (just shy of coil bind) and squeezed off the first shot.... 873 fps with the 77.8 gr. EunJins.... 132 FPE !!!    The next shot was a bit slower, and then a further drop on the third shot to 838 fps, after which I was down to 2100 psi.... 300 psi per shot, WOW! this thing is using a lot of air!.... I had wondered if the hammer setup I had would be enough to dump 3000 psi, and the answer was yes !!!

I tried backing out the hammer spring two turns, and the shot string started out at only 759 fps, then climbed to 835.... Hmmmmmm.... not enough hammer hit for 3000 psi.... I then tried in between, and at 1 turn out got a nice bell curve of 4 shots.... The results of the testing are in the graph below....

(http://i378.photobucket.com/albums/oo221/rsterne/9mm%20PCP%20Carbine/9mmat3000psi.jpg)

At maximum, 3 shots, falling velocity, averaging 127 FPE, 900 psi used, and the efficiency was 0.77 FPE/CI....
At 1 turn out, 4 shots within 3%, averaging 121 FPE, 1000 psi used, and the efficiency was 0.88 FPE/CI....
At 2 turns out, 5 shots within 5%, averaging 115 FPE, 1100 psi used starting from 2800, and the efficiency was 0.94 FPE/CI.... VERY NICE !!!

I now have the ability to set this gun up for any fill up to and including 3000 psi.... I can set it up for a 2000 psi fill and get about 9 shots at 80 FPE.... all the way up to a 3000 psi fill and about 4 shots at 120 FPE.... or anything in between.... It appears that between 100-120 FPE I will get about 5 shots, with higher fill pressures resulting in more power.... IMO, this represents the "sweet spot" for the gun, filling it between 2400-2800 psi and getting velocities between 760-840 fps.... Throughout that range, the Efficiency plateaus at about 0.94 FPE/CI.... Above that range, the efficiency and shot count drops.... Below that range, the efficiency and shot count increases....

This gun is now shooting at up to 6 times the FPE level of a stock Disco.... I'm a VERY happy camper....  ;D

Bob

Title: Re: 9mm and .308 cal Disco Build
Post by: Rescue35 on January 28, 2012, 11:16:32 AM
Very well done sir!  ;D
Title: Re: 9mm and .308 cal Disco Build
Post by: grumpy on January 28, 2012, 03:01:22 PM
Thats a brute ! Thinking of offering a kit ?
Title: Re: 9mm and .308 cal Disco Build
Post by: rsterne on January 28, 2012, 03:20:53 PM
Here is all the data from my testing on one graph.... It makes it really easy to see how the number of shots decreases as the power increases.... and how the gun uses more air per shot (slope of the pressure lines)....

(http://i378.photobucket.com/albums/oo221/rsterne/9mm%20PCP%20Carbine/9mmVelocityCurves.jpg)

Interestingly, from 2 to 4 turns out on the adjuster the gun produces 5 shots, with little change in efficiency.... just a higher fill pressure required and a slightly wider range of pressure used.... It looks like with further testing I can fine tune it for 5 shots between 800 and 840 fps (5% variation) using a 3000 psi fill and about 1.5 turns out on the hammer....  ;D

There isn't much more I can do on this gun until I manage to size the cast bullets appropriately for both this barrel and the .308.... I may work on the chamber of the .308 barrel next to see if I can get it to chamber the rounds properly.... I would still like some advice on the proper way to make a chamber for cast bullets if someone can help with that....

For those that don't know, I never build guns or parts for others.... I can't even find enough time to do all the projects I have on my plate now.... Your only chance to get one of my creations is to wait until I need to sell one to pay for parts for the next one....  :o

Bob
Title: .308 Disco - First Tests
Post by: rsterne on January 30, 2012, 12:30:14 AM
I spent the afternoon chambering the .308 cal. barrel for this gun.... I drilled 5/16" to a depth of 0.6" past the front of the transfer port which gave me a parallel chamber that the bullet would slide nicely into.... that was the easy part!.... I didn't have a chambering reamer, nor any reamer with a 1 degree taper, and I needed to taper the end of the rifling from the top of the lands (0.300") to the bottom of the grooves (0.308") over about 1/4" of length.... I machined up a piece of steel bar to the appropriate dimensions, coated it with a very fine abrasive paste, and with the barrel in the headstock and the tool in the tailstock, I turned the chuck by hand while applying pressure with the tailstock handwheel.... This was a VERY laborious process, it kept getting stuck, and at one point I succeeded in welding the pieces together and had to drive the tool out with a 1/4" brass rod.... It took me a couple of hours of fiddle-pharting around, but I finally got a decent leade on the rifling.... When you chamber a bullet and then push it back out, you can see a smooth, tapered imprint of the rifling for about 2/3rds of the front driving band on the bullet, which was sized to 0.308".... 

After dinner, I just had to put a few rounds through it, of course.... I had two different bullets, both Lee that I cast myself, one a 120 gr. Round Nose and the other a 113 gr. Flat Nose (which actually weighs 118 gr.).... The two bullets are virtually identical except for the flat.... I had some that were as cast (0.309"-0.311") and some that had been sized to 0.308" with my Lee press.... Cast bullets always seem such a tight fit to me.... I tapped a sized bullet into the barrel and then drove it back out (it was too tight to push through) and the rifling was deeply engraved, right down to scraping slightly on the grooves of the bullet in some spots.... There was no sign of the driving bands being shaved by the bore between the rifling however, so I decided to try them out.... They chambered easily, especially the sized bullets.... My SCUBA tank was down to 2900 psi, but I shot a few groups, enough to at least find out where I'm at.... Here are the results....

(http://i378.photobucket.com/albums/oo221/rsterne/9mm%20PCP%20Carbine/308DiscoVelocitiesjpg.jpg)

The first string I shot with the hammer spring preload at maximum, just shy of coil bind.... and was rewarded with four shots, the third of which was the fastest at 685 fps (123 FPE).... but the 5th shot was only 598 fps, and when I checked the pressure remaining, it was only 1400 psi.... 300 psi per shot, an efficiency of just 0.73 FPE/CI !!!.... I backed the preload out 1 turn for the rest of the testing, and to my delight, just that small change dropped the air usuage to 250 psi per shot and gave me 5 shot strings starting at 2800-2850 psi and ending at 1550-1600.... The 118 gr. and 120 gr. bullets shot nearly the same velocity, so the heavier one had a tiny edge in FPE.... The sized 120 gr. averaged 112 FPE and the 118 gr. were 110 FPE.... The efficiency was about 0.80-0.81 FPE/CI.... The unsized bullets were a LOT slower, losing 40-48 fps, so they were obviously way too tight in the bore.... The 120 gr. was only 99 FPE and the 118 gr. just 96 FPE.... and the efficiency was down to 0.70-0.72 FPE/CI.... The dotted lines in the graph show the unsized bullets....

I'm actually quite pleased with the results using the sized bullets.... I expected the FPE and the efficiency to drop because of the smaller bore.... but the heavier bullets, with their much higher SD, gained back much of the loss.... At the same hammer spring setting, the 9mm only had about 9 FPE more energy and about 14% more efficiency.... I do have a bit of a leak at the nose of the bolt to deal with, so when I fix that, I may find that the .308 gains a bit more.... In addition, I hope to get a smaller sizing die and try the bullets at 0.306"-0.307" and see if the velocity increases.... In any case, the gun is now interchangeable between .308 and 9mm....   ;D

Bob
Title: Re: 9mm and .308 cal Disco Build
Post by: Rescue35 on January 30, 2012, 09:00:24 AM
Again, nice work Bob. I really like the fact that you graph it out in excel and are willing to share it with us.
Title: Re: 9mm and .308 cal Disco Build
Post by: airpuffhunter on January 30, 2012, 09:53:06 AM
congratulations, that gun is really something.
Bob I wonder, can you brake the valve, if the hammer hit it too hard? (too much preload)

Title: Re: 9mm and .308 cal Disco Build
Post by: airpuffhunter on January 30, 2012, 09:57:59 AM
another thing, do you think that if you use the same weight bullet in 9 mm, will get a lot more power?
Title: Re: 9mm and .308 cal Disco Build
Post by: rsterne on January 30, 2012, 12:39:54 PM
I guess you might shorten the life of the valve if you hit it too hard with the hammer....

Generally, in the same gun, set up the same way.... if you keep the same bullet weight but in a larger caliber you will get higher velocity and energy.... This is because the base of the bullet (that the air pushes on) is larger and hence more force is available to accelerate it.... There are limiting factors, of course.... If the bore is too large in volume compared to the valve you may not have enough "oomph" for the larger bullet.... I would certainly expect that if I used a 120 gr. bullet in 9mm that the velocity would be higher than the same bullet in .308.... and the energy higher.... PCPs tend to favour heavy bullets and develop more FPE with them.... I have a 132 gr. bullet in 9mm but they are too large and I need to size them down to work properly.... The first trial (at 2000 psi) resulted in slightly less energy than the 78 gr. EunJin at the same pressure....

Bob
Title: 143 FPE !!!
Post by: rsterne on January 30, 2012, 07:52:10 PM
After yesterdays success in chambering the .308 barrel I decided to rechamber the .357 barrel today for cast bullets.... The process went better than yesterday, and when I put the bigger barrel back on I got the following results....

(http://i378.photobucket.com/albums/oo221/rsterne/9mm%20PCP%20Carbine/9mmCastBullets.jpg)

As I expected, this combination gave the highest FPE levels I have achieved so far.... the peak level with the 132 gr. Lee Round Nose was 698 fps which is 143 FPE.... With the hammer spring at maximum preload I got four shots using 300 psi per shot.... The average was 687 fps (138 FPE) at an efficiency of 0.84 FPE/CI.... I backed the preload out 1.5 turns and got 5 shots starting at 2800 psi and ending at 1550 (250 psi per shot).... They averaged 650 fps (124 FPE) at an efficiency of 0.90 FPE/CI....

The new longer chamber cost me about 10 fps with the EunJin pellets at the same spring preload.... However, by winding in the spring 1/2 turn and increasing the fill pressure to 3000 psi I gained that back and still kept 5 shots at 250 psi per shot.... So the current results are 5 shots at an efficiency of 0.90 FPE/CI with either pellet/bullet at the same 1.5 turns out on the hammer spring.... The 77.8 gr. EunJin shoots 812 fps (114 FPE) using a 3000 psi fill, and the 132 gr. Lee shoots 650 fps (124 FPE) filled to 2800 psi....

I'm pretty darn pleased with the results, and I certainly learned a lot building this Big Bore Disco.... Who would have thought you could take a gun all the way from 22 FPE to 143 FPE....    ;D

Bob
Title: Re: 9mm and .308 cal Disco Build
Post by: lloyd-ss on January 30, 2012, 08:32:31 PM
Bob,
Again, very nice.  Bullet fit has always been illusive to me, too.  I have actually taken a piece of 1/2" steel plate and drilled and and reamed a series of slightly undersized holes, and then gone back with a tapered pin reamer and opened up each hole slightly more than the previous.  That way I had a nice assortment of sizes for testing.  Then once I got the size figured out, I worked on a real sizing die to produce the same bullet diameter.

143 fpe from a Disco configuration is sweet!
Lloyd