Disco Double Carbine

[BC81]

Hello all, just wanted to post up a new project im building. Its a disco double tube kit i made and mounted to a 22xx grip/carbine stock. Ported valve and using Prod gauge block. Ported barrel also, she is slinging kodiaks @ 840-860 fps for about 28 good shots. Just need to finish up the barrel band and make another. Enjoy!

[David Mccann]

cool. any detail pics of your homemade parts?? id like to have a setup with dbbl. tubes. cheers

[BC81]

Pretty simple really. Just used 2 Prod gauge blocks and a SS 1/8" npt close nipple attaching the tubes. Had to cut it down and rethread it to get the length i needed. The bottom tube is a disco tube cut down to fit past the grip. It will clear the rifle trigger too, i just need inlet the stock. I plugged the back of the bottom gauge block with an npt socket style pipe plug. Tubes are holding 2800 psi no problem. Used an extra fill valve for the front and cut up 2 barrel bands to make one. Its JB welded together for now till i get 2 fresh ones machined on my lathe. No gauge yet, will probably drill and tap the bottom of the gauge block for that. The valve has been upgraded to 10-32 socket screws 180K tensile strength.
Pretty simple really, the barrel bands are the hardest part of the whole deal.

[farmerjoe99]

Sorry if I missed it but how did you seal the rear of the second tube?
interesting idea to use another disco tube.
just be careful and make sure everything thing is rated for
well over what pressure its going to see.

[BC81]

I used a Prod gauge block in both tubes in the gauge location. On the bottom tube at the back of the gauge block i drilled and tapped it for 1/8" npt and used a socket head plug rated for HPA. The stainless nipple between the two was shortened to about 1/2" of threads on each side and rethreaded. Its rated for 9000 psi.

[FuzzyGrub]

FWIW: I made a DIY double tube Prod in a similar way, about 2 tears ago.  I have not had any issues with it, but that does not mean it is 100% safe.   And never posted any details on this mod, because I wasn't sure.  While the fittings and gauge blocks should be fine (Note: I did not shorten the nipple), there is an unknown at the bottom tube gauge block.  The only thing holding it in is the nipple against the side/edge of the tube.  It puts the nipple in sheer, but I believe the weak point is the tube.  I have not seen any bulging on the tube as of yet, like what Lloyd showed in his disco tube/valve screw test.   So now that this mod is out in the open, sort of speak, maybe someone can run the numbers on that connection.  I also think there can be ways to take the load off the nipple / tube edge.

It also takes a bit of practice to get the tubes sealed.  You have to get tight connections right at the point where the two tubes are parallel.   You also need to pressurize to around 500 psi and adjust position of the tube (gauge block back and forth and side to side slop).   

Now, as I mentioned, I didn't cut the nipple so have about a 1/4" gap between tubes.  NPT is tapered threads and wanted to get as much threads engaged as possible.   The barrel band needs to be custom made to fit and maintain that gap.  This was done with a shotgun style tri-rail.  The gap does allow for mounting of the 1" gauge on the front. 

While I did provide more details here, it is mainly for those that are going to do this anyway.  If you want to be safe, just buy Lloyd's kit.  He has designed all the safety factors in.

[BC81]

Problem is Lloyd's kit is unavailable at this time and im impatient lol. I plan on anchoring the bottom gauge block with 2 or 3 10-32 set screws, similar to how the valve is mounted. Should be plenty safe then. Also, the tube shouldnt move or bulge since the upper gauge block is attached amd butted against the valve in the upper tube which is supported by the 180K 10-32 cap screws.
Also, this setup ran me about $80 in parts and a few hours of tinkering. I will eventually switch it to the wood stock and add the gauge how i had mentioned earlier. The plan is to have this rifle for hunting medium sized game and be able to hunt all day via the 25-30 shots on board @ 35 fpe. She does dime sized groups at 30 yards or so, perfect for a day of groundhog hunting! I don't have a portable tank and dont want to bring the hand pump hunting. However, this thing takes about 200 pumps to fill now lol
As for the gauge, i could remove one of the fill valves foster fittings and check valve and mount my mini 3000 psi gauge up there but @ $20 a pop, I'd rather not take the chance of damage or a leak. That and i like both fill caps up front. Also, i can fill from either fittinf which is nice if one clogs up. A do have a TKO for it as well, just need to mod one of the caps.

[BC81]

So i checked so figures. 316 stainless has a maximum tensile strength of 84,100 psi and a yield of 42,100 psi. Factor in that most shear strength figures are 60% or so of the yield strength, that means the shear value is about 25,260 psi. Im going to say thats a gracious figure but well above the 2500 psi against it. Also being such a short section gives it less leverage against it, i would say its ok.

 Then again, im no structural engineer or metalurgist so take what i found on the googler with caution if attempting this mod!
IM NOT LIABLE!!!

[FuzzyGrub]

I'm not either, and why I raised the caution flag.   

Some of the info you would want to review is in this thread: http://www.gatewaytoairguns.org/GTA/index.php?topic=25091.40

The nipple on the bottom tube is basically acting like a single valve screw, be it a large one. 

Machining flats on the gauge block and match drilling to the tube was one of the ways I was thinking that could alleviate the issue. 

[BC81]

Thanks Fuzzy, ive read through most of these threads on all the safety stuff. This is my 4th pcp build on the disco/22xx platform. I pretty much followed in the shadows pn all of Bob Strene's posts.
Ive already upgraded my valve screws and turned the heads down to fit perfectly in the tube holes so the valve doesn't move, unlike my standard disco i inletted to a AR2078A thumbhole stock. Those are pushed back in their holes, but are also 180K strength so i can fill it up a lil higher. I have had both of these guns up to 2800 psi without issue. I habe .040" springs with RVAs on both. Also have a 14.5" 2260 pcp pistol/carbine i built, also 180K screws flush with the holes.
Safety is definitely a concern, i have lil ones in the house and i keep m guns filled. They're in a hardcase but never know, HPA is very dangerous!

[BC81]

Here is the 2260
She is setup for 20 shots @ 15 fpe with CPHP or 15 shots @ 20 fpe with JSB 18's

[rsterne]

The tensile or shear strength of a material is NOT the same as the pressure it can sustain.... You need the cross-sectional area of the part under stress, to calculate the stress in the material.... then you need to compare that to the load on that part....

The load is pretty easy.... A Disco tube is 0.745" ID, which is 0.436 sq.in.... at 3000 psi, the end force on the gauge block is therefore 1307 lbs.... For a safety margin of 3.5:1 (recommended) the part(s) locating it in the tube need to withstand a force of 4575 lbs. without breaking....

My understanding is that shear strength is 60% of the UTS, not the yield, so that would be 50,460 psi.... To withstand a shear force of 4575 lbs., you would need a cross-sectional area of (4575 / 50,460) = 0.0906 sq.in. of 316 stainless steel....

I wasn't able to find the root diameter of a male 1/8" NPT thread at the edge of the female thread that it screws into specifically, but by subtracting 0.8 x the pitch (standard thread height) from the pitch diameter at hand engagement (0.374") I get 0.344".... If the hole through the fitting is 3/16" (typical), then the cross-sectional area is (0.0929 - 0.0277) = 0.0652 sq.in.... That is below the area calculated above for a 3.5:1 safety margin in shear at 3000 psi.... The 3.5:1 safety margin would occur at a fill pressure of 2160 psi, and ONLY if the material and dimensions agree with those used for the calculations.... Incidently, the UTS I found for 316 SS is only 75Ksi....

The other thing to look at is the load on the tube wall.... Assuming the OD of the nipple is round, and a good fit in the hole in the tube, this can also be calculated.... If the OD of the nipple is a hex, or fitting in an oversized hole in the tube, there will be a point loading that could be many times higher.... If we assure the OD of the nipple is 1/2", in a 1/2" hole, then the area of the tube under load is (0.5 x 0.065) = 0.0325 sq.in.... For 1018 DOM steel tubing, the yield strength is 54,000 psi, so the force required to start distortion in the tube wall would be 1755 lbs.... That would occur at 4025 psi, which means the safety margin to yield on the tube wall is less than 1.5:1, definitely NOT recommended.... As I said, if the nipple and hole do not match, a much lower pressure would be expected when distortion first occurs.... In addition, 316 SS is softer than 1018 steel (it has a lower yield strength), so we could expect the nipple to distort before the tube....

As I have said many times before, I am NOT an Engineer, but I personally would NOT allow a 1/8" NPT pipe nipple to sustain the end force on a gauge block as described above....

Bob

[BC81]

Thanks for chiming in Bob. So do you think if i reinforced the gauge block with 3 10-32 180K set screws and lowered the fill pressure that it would be in the safety margin? I'd even be ok with it at a 2:1 ratio. The weak link is the nipple, which is going to be inside the wood stock when all is said and done. So if it failed it would have to blast through that as well. Just a thought, obviously not ideal but it will only be in my hands in the field, it aint my backyard or basement plinker!
Also, the 1/8" nipple is rated to 9000 psi, i got it from granger if that helps.

[BC81]

I went ahead and degassed it for safety and to check for distortion, also to do the safety screws and drill/tap for the gauge. I did however shoot it over the chrony with kodiaks at full spring tension and wow... it did 20 shots in the 930-960 fps range! And still had air but shot 21 was down in the 890s. Im a happy camper with that... capable of over 40 fpe!

[rsterne]

As per the information in Lloyd's video, three 10-32 high tensile screws, with the heads set down through the tube, are OK for at least 3000 psi, with a suitable safety margin.... You want the load on the screws, NOT on the side of the nipple....

The pressure rating on the fill nipple (9000 psi) has nothing to do with putting it in shear to locate the gauge block.... totally different things....

Bob

[BC81]

I get that. Well i will be tearing it down tomorrow after work and fitting the screws the same way i did the valve. I can see the nipple hasn't shifted into the side of the tubing, but the gauge block being pushed by the pressure is putting thw threaded area of the nipple in shear. Thats what you're meaning, correct?

[rsterne]

Correct, there is a shear plane right where the nipple threads into the gauge block.... and even a bit of leverage on it as well, because that point is slightly further in than the ID of the tube that the nipple is pushing against.... No way to calculate the additional stress from that, however.... Can you see any dents in the side of the nipple where it is contacting the tube?.... or marks in the back edge of the hole in the tube from the nipple threads?....

This only applies to the bottom tube, of course.... because the upper tube has pressure on both sides of the gauge block, and no net end force on it.... the nipple just floats in the hole, even under pressure....

Bob

[FuzzyGrub]

I can see the nipple hasn't shifted into the side of the tubing,

  That surprises me.  I'd expect that the nipple would be pushed against the side of the tube even if it only had 500 psi.  Did you check that before or after depressurizing? 

[BC81]

Yup, i checked it. I tore it down after dinner and all seems well. The threads are in good shape still, no dents or anything. I only had it filled to 2000 psi overnight. I guess since the top block was seated against the valve and with it being centered in its holes, there isn't any play in the screw holes, that it helped hold it centered. I think i got very lucky. I checked it pretty thoroughly and didnt notice anything wrong with it. I wonder if the shear force got applied to both sides of the threads vs just the one end and maybe it lessened the load. Either way, it's getting drilled and tapped for the 180K 10-32 shcs tomorrow and i will start working on the stock Inletting. I will take progress pics tomorrow of the block, tubes, and nipple. Hopefully i will get it back together and will post a few chrony pics of its results with the kodiaks and eunjin domes.
Forgot to mention, the nipple has an 1/8" raised section in the middle separating the 2 threaded sides. The side thats on the lower tube threads deeper and that raised section i believe is what saved me. It must have been rested against the tube. Its easily another 1/8" thick of material, probably helped stop the sideways movement. I'll take a pic tomorrow and show what im meaning.

[rsterne]

As I said, in the upper tube, the gauge block has pressure on both side, no net force, the nipple should just sit loose in the hole.... You do NOT need screws in the top gauge block....

The LOWER tube is a totally different story, because the back end (where you plugged it) has no air pressure against it.... The lower tube should slide forward until the nipple pushes hard on the back of the hole in the tube.... THAT is where you need to look....

Bob