9mm and .308 cal Disco Build

[rsterne]

I figured I should post my current project in this forum.... I'm moving files over from another forum, so be patient.... Here's the start of the thread....

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....



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....



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

[rsterne]

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

PS - This gun will be tuned both ways, under 500 and whatever I can get....

[rsterne]

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

[rsterne]

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....



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

[SeanMP]

[QUOTE=rsterne;1234

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[/QUOTE]
Bob

The common numbers we used to toss around in BR shooting was the the throat needed to be 0.001-0.002 larger than the bullet and the leade needed to be 1/2 to 1 deg

The tight tolerance on the throat ensure that it aligned the round as it entered the leade. Any sharper than 1deg and you could suffer accuracy loss from shaving

HTH

[rsterne]

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)....



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....

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....

Bob

[rsterne]

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....



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....



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

[rsterne]

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....



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....



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....

Bob

[rsterne]

After last nights discussion on another Forum about the strength of the Disco valve securing screws, I decided to make a jig for drilling a Disco valve for 10-32 high-tensile screws.... Here are the photos....



The jig is made from a piece of 1" x 1.5" x 2" aluminum.... A block of steel would be better for production work.... I drilled a 3/4" hole 1.5" deep and then ran a 5/8" mill down along the bottom side and removed 0.015" to form a shallow groove (ie the hole is slightly "pear" shaped).... This forms a "V" block so that when the valve is inserted it only touches at two points.... On the opposite side is a 3/8" SHSS with the end ground flat so that it won't damage the valve when snugged up.... There are two holes drilled in the top beside the setscrew.... The front one is a #21 drill (the correct size for tapping a 10-32 thread) and the rear one is a #11 drill (just clears a 10-32 tap).... There are two brass pins, the smaller one has the end machined to 0.128" which just fits in the 8-32 tapped hole in the Disco valve.... The larger one has the end machined to nicely fit in a #21 drilled hole....



To use the jig, you slide the rear valve half in until one of the screw holes lines up with the front hole and then slide the small pin down until it locates in the 8-32 thread hole.... Then you snug up the 3/8" setscrew to hold the valve in place and pull out the pin.... If you were doing a bunch, you would set the block up in the mill, aligning the hole with the head.... but I just used a hand drill and drilled by eye, using the hole in the block to guide a #21 drill to clear out the threads in the valve.... I used a normal drill, very carefully, to just remove the threads, making sure the point didn't dig in as there isn't much meat between the bottom of the hole and the inside of the valve.... I then changed to a drill ground flat on the end to make the hole full depth with no danger of cutting through.... This operation could also have been done with a 5/32" end mill provided you stopped at the correct depth....

I loosened the setscrew, removed the valve, and the threads were evenly removed, so I turned it around and replaced it in the jig and slid it in until the newly drilled hole lined up with the rear hole in the jig.... I slid in the larger pin to locate the valve and snugged up the setscrew again....Using a 10-32 plug tap, with the hole as a guide, I tapped the hole until the tip of the tap just touched the bottom (about 3 threads).... I then changed to a bottoming tap, and got nearly 6 threads total.... I repeated this for all three holes and then ran a countersink into the top of each hole in the valve just to provide a slight relief because the thread on the screws stops a few thou from the head....

I had to shorten the 1/4" long low-profile 10-32 screws to 3/16", properly finished the ends, checked the thread in a nut (rather than risk galling the aluminum valve), and then installed them to make sure they went all the way down and that the head was seated on the flat on the valve.... Everything worked perfectly, so it was time to install it in my 9mm and give it a try.... The screws I am using are rated at 180,000 psi tensile strength, which means 108,000 psi in shear.... That will give me a 4.4:1 safety margin at 3000 psi, making the screws no longer the weak link in the Disco.... The OD of the head on the screw is a few thou smaller than the holes in the Disco tube, but the head still seats well enough to spread the load and the transfer port still lines up as good as they ever do....

Bob

[SeanMP]

That's a brilliant solution Bob. If you have any bore paste (JB Bore or equivalent) you could whip up a snug fitting mop and work the throat and leade a bit to align the machining scratches and refine the finish.

BTW that string looks excellent!

Post a pic if you can of one of the chambered bullets, I'd love to see it.