Air Gun Home :: View topic

rsterne · Moderator

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

domer_pyle · Senior Member

Awesome! You planning on ever taking it over 500 fps?

rsterne · Moderator

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.... Mr. Green

radar · Silver Status Member

Stock bullets or are you going to cast? What weight are you thinking?

rsterne · Moderator

For the 9mm I'm using 77.8 gr. EunJins although I may try some cast ones later.... I have a couple of Lee molds (113 gr. & 120 gr.) and a .308 sizing die when I get around to the .30 cal since you can't buy pellets....

Bob

radar · Silver Status Member

Interesting project. Keep us posted.

rsterne · Moderator

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

radar · Silver Status Member

Are these the 30 cal, or the 9mm? I wonder if, weights being equal the 9mm would put out more fps/ft. lbs. because of the higher possible theoretical efficiency of the larger bore...That is more sq. in. of bullet = more pounds force acting upon it, if the valve can keep up. Question

rsterne · Moderator

All testing so far is with the 9mm EunJin 77.8 gr. pellet.... I agree that larger bores tend to be more efficient because of the larger base area (and hence more force available).... provided the valve can keep up.... and the frictional losses between the pellet and bore are not significantly greater....

Bob

radar · Silver Status Member

My understanding is that though the base area, and therefore lbs force, increases as a function of the square of diameter, the barrel friction increases only as a direct function of diameter, which makes sense if you stare at the pi*r^2 and circumference formulas for a second. Maybe this is why hybrid big bore spud guns have broken the sound barrier using sabots with relatively low pressure and large air+fuel volume...(and a long barrel).

So you are definitely making progress. Congrats on your latest velocity figures! You are, so far, hitting about Daisy 22SG velocity with a huge pellet, that would hit hard, indeed.

This is fascinating stuff, and thanks for keeping a running report. I'm finding I'm looking forward to checking in on this project for new developments.

-Rob

rsterne · Moderator

While your math is correct regarding the area/cirfumference relationship, it doesn't account for bullet fit, which is why I added the proviso in my post..... The 9mm EunJins are quite a tight fit in the barrel, even though the barrel is 0.357".... They were even tighter in the 9mm barrel I had on my Carbine.... Getting the right bullet "fit" is the bane of Big Bore airguns.... Too small you get blowby.... Too big you can't load them or you lose a ton of velocity.... From what I've read, once you start using flat based bullets instead of pellets with an expanding skirt.... you have to have the size within 0.001 or less of ideal.... That and the hassle of casting (and sizing) your own bullets is one reason they aren't very popular.... I expect those problems with my .308 barrel because even a bullet sized to 0.308" is too tight to load properly.... I pushed a sized bullet through the bore and it only showed rifling marks, no shaving by the grooves of the barrel.... but I had to drive it out the same end I pushed it into, I couldn't even push it through....

Bob

SMP · Silver Status Member

What about a hybrid.

If the last driving band of the boolit was thin walled would it not obdurate to seal the bore?

rsterne · Moderator

I'm glad this discussion is on this forum and not on the GTA Big Bore Forum.... What I am about to suggest would be considered sacriledge over there....

Why don't big bore shooters use large, heavy pellets?.... I mean complete with a hollow base and a skirt that can obturate properly.... They go into long diatribes about lead hardness and swaging to 0.0005" tolerances and how handgun bullets of various designs work.... and then ignore the fact that even though the velocities of high power big-bore airguns are similar to PB handguns.... the pressures are WAYYYYYYYYYY less....

Here is an example of a pellet design that is twice the weight of a roundball, regardless of the caliber chosen.... It would weigh 88 gr. in .308, 136 gr. in 9mm and 286 gr. in .45 cal....

There is no reason that current pellet technology can't be extended to big bores.... All it takes is the will and the money.... Incidently, in .177 cal it would weigh 16.6.gr, in .22 cal it would be 30.6 gr. and in .25 cal it would be 46.8 gr.... ie in the range of the heaviest pellets currently available.... In addition, the design is "scalable" by simply lengthening or shortening the cylindrical section in the middle.... Mr. Green

Bob

radar · Silver Status Member

That looks like the JSB Match Diabolo Exact Jumbo Monster.
I don't know why the big bore boolits are so similar to regular firearm dittos, although the air venturi ones I use are for the airguns of course, and have a bit of a hollow base, but not so much as these or a minie ball.

This design you show here looks more appropriate.

SMP · Silver Status Member

Agreed....In fact that cross section looks remarkably similar to the Prospero field line Wink

What would be the result if you took one of your standard cast boolits and back drilled it to the first driving band, to thin the wall sufficient to obturate, and then a pass through the sizer for full engagement on the rifling but not a full seal so the frictional forces would be limited to the rear most band