.30 cal Disco Double

[Rdsail]

Bob,

Very good point. I think I will stay away from that. I'm just going to get a second valve taped for 10-32. Then I will have both options. I got the prod gauge port on the mail today. What a difference in quality? I installed the McMaster 0-4000 gauge to try it out.  I'll gas it up tonight for a pressure test.

Daniel

[rsterne]

HI Lloyd, thanks for chiming in.... much appreciated.... I used 1010 mild steel with a 44K yield in my calculations, and with three screws all taking equal load, I get a 0.150" contact width per screw required at yield at 3000 psi.... in other words, if the yield strength is 44K, with 3 screws, they in theory should "settle in" and stop moving after that.... Using a 0.312" hole and a 0.270" head, that means the screw has to move 0.004" aft.... for a total of 0.025".... This is assuming, of course, that all the holes in the valve and the tube are drilled in EXACTLY the same plane, which is VERY unlikely.... I didn't duplicate the calculations using 70K for the compressive strength of the tube because I don't know what material it is.... but CrMoly is only 63K yield IIRC?....

Anyway, I overestimated how far the valve is likely to move back, but since I've seen a fractional settling in even with a 5/16" head size on the 10-32 screws at 3000 psi (I thought you had as well?).... I think we can assume that the valve might, on average, end up about 0.025" further back than it should.... but we can also assume that once it has settled into it's new position it should be perfectly safe.... wouldn't you agree?.... Sorry if I first erred on the side of caution....

Bob

[rsterne]

Rdsail.... Yes, the PRod gauge port is a lot bigger thru hole, but it uses 1/8" NPT threads so does not accept the Disco gauge adapter which is straight thread sealed with an O-ring....

On reflection, I don't think there is any problem running the 0.270" OD head size on the high tensile screws.... but I would move the center of the valve exhaust port forward about 0.025" to allow for the difference in diameter and any "settling in" with the smaller screws....

Bob

[Rdsail]

bob,

I air up the disco and the McMaster gauge seams to be reading 100 psi lower than actual. The only thing I wish it was different for the needle to be pointer. I'll let it site over night and see if there are any leaks. I filled to 2200 psi for the pressure test. I can't wait to test it tomorrow.

Lloyd,

thank you for all the help!

Daniel

[rsterne]

I got the gun assembled with the short .308 PB barrel and fired a few test shots tonight.... First the photo.... I know, it's ugly with the barrel shorter than the tubes.... and parts still needing bluing.... but you can see the potential there....



The gun was pretty hard to cock, I'll have to take a look to see if there is anything binding.... However, I was still able to cock it at coil bind, and the good news is that is way too much preload, even at 3000 psi.... The first shot was the fastest, the gun was very loud, and the 46 gr. JSB pellet screamed through the Chrony at 971 fps which is 96.3 FPE.... Four shots later it was down to 920 and those 4 shots used 700 psi.... I backed the adjuster out 2 turns, and got 6 shots starting at 953 and ending at 918, using 750 psi.... Out 2 more turns, and I was starting to get a string.... Starting at 2900 psi (my tank needs filling) I got 10 shots starting at 914, peaking at 929, and ending at 886, and it used 800 psi....  Backed out 2 more turns, starting at 2900, I got 13 shots starting at 861, peaking at 906, and ending at 860 fps at 2000 psi.... That string averaged 880 fps (79 FPE) at an efficiency of nearly 1.1 FPE/CI.... At 8 turns out the usuable pressure range is from about 2600 down to 1600, returning 14-16 shots in the mid 800s.... and at 10 turns, the string starts in the high 700s, peaks in the low 800s, and returns, but this is with running stock Disco pressures starting at 2000 psi and ending at about 1300.... and the gun is still shooting 65 FPE, three times what a stock Disco does....

These are just preliminary tests, using upsized JSB 46 gr. pellets in a 21" barrel.... I just couldn't stand the suspense of waiting until the new barrels get here.... The additional barrel length should add roughly 5 FPE.... but I have already surpassed my goal of 88 FPE in .30 cal.... so tonight's testing was a resounding success.... BTW, I was running a 3/16" transfer port, I can go to 7/32" if I want to....

Bob

[Rdsail]

Great Job Bob!

It is amazing the numbers you are getting out of the disco. It is going to be nice once you get the barrel in and start shooting.

Daniel

[Rdsail]

Bob,

I shot some strings. I did not record them but I gained about 50 fps on the same power setting as before. (4 turns from binding point) The only thing I noticed is that It shots are slow on pressures over 2050 psi, Significantly slower. The valve seams to close too fast over those pressures. My shot range in relation to psi went up before I could here the hammer bounce starting at 1450 psi now it starts at 1200. So it starts wasting air at a lower psi. My high before was of 848 now my high is 898. I might have to tune it down some. I gained about 2 shots over all taking into consideration that my PSI range went up. The deviation went up.

What this tells me is that I might not have to fill over 2000 psi after all. I'm going to try my short heavy spring (-2.5 coils). I alway got a better deviation with the shorter spring.

Any ideas on how to increase my speed over 2000 psi? Heavier hammer? I don't want a heavier spring because the cocking will get harder.

I was not shooting for accuracy right now but from 15 yards I'm getting a ragged hole. I was not trying to shoot well. Just looking at chrony for numbers.

Thanks.

Daniel

[Sqrl Klr]

Sweet. Congrats Bob.

[rsterne]

Daniel, it's a bit hard for me to keep track of what you are doing, so difficult to offer advice.... The details of your project get buried pages back in the info on my .30 cal (eg. I don't even know what caliber your gun is or the pellets you are shooting).... Perhaps it might be better to start your own thread, giving the details of what you have done with the results at each stage so that I can follow it easier?.... Generally, a heavier hammer and a stiffer spring or more preload all act in a similar manner.... Increasing any of them allow a higher pressure before the velocity drops off.... Increasing the pressure (providing you have enough hammer strike) will allow the gun to produce more power.... It's a matter of deciding "how much is enough" and what pressure range you want to (or can safely) run.... My .30 cal can be tuned in a variety of ways, with higher pressure producing more power, and likely fewer shots.... Once completed, I'll have to decide where I want to tune it.... I typically go for as much as I can get (within the safety limits of the platform) and then detune it for he results that make sense, balancing power vs. shot count, with an eye to reasonable efficiency.... which for me, means striving for over 1.0 FPE/CI....

Bob

[Rdsail]

Bob,

sorry for highjacking your thread....I'll start a new one but I'll keep following this one. I'll leave with one note. My efficiency went form 1.06 to 1.42. I could not believe the numbers the only thing is that I don't get as a flat curve as I used to.

Thanks for the Help.
Daniel

Gauge note: the mcmaster gauge does not read as well as the disco one. 100 psi low. I'll put the old one back on on the next tear down.

[Rescue35]

Nice work Bob. I love it when hard work pays off.

[venxxxxx]

Very inspiring  . Now test it on a yote?? 

[lloyd-ss]

Very nice Bob, and a good range of power levels to choose from. 
Seeing that you ended up shooting at 6 to 10 turns out, I can see why it was hard to cock at coil bind, LOL.

A well thought out project!

Lloyd

[rsterne]

I changed the gun over to a rear cocker last night, and also enlarged the transfer port to 7/32".... Here is the hammer, which now weighs 82 gr. with the cocking knob....



The gun is now very easy to cock.... I put a small O-ring on the shaft which can be used to directly measure the valve lift.... You simply slide the O-ring up against the back of the RVA screw, cock the gun, fire it, and the O-ring slides back exactly the amount the valve opens and you can measure the space with calipers.... I did that for all RVA settings from coil bound to 12T out, for the first shot, and the last shot before the velocity fell more than 4% below the peak.... The smallest lift measured was just under 0.06", and the largest was 0.13".... In virtually every case where there was a proper bell-curve, the lift at the first shot was about 1/16".... and at the last shot was less than 1/8".... This is a very valuable piece of information, and while it won't be the same for every gun, for this gun it indicates that limiting the valve lift to 0.15" (eg. by having the hammer hit the back of the valve at that point) would not have any affect.... That means I can lengthen the hammer stroke on this gun to a full 3/4" by grinding a bit more off the valve stem or counterboring the front of the hammer a bit more.... The fact that the valve lift was virtually the same regardless of pressure and preload.... once the gun was adjusted for a proper bell-curve.... was a bit of a revelation to me.... though in retrospect it makes perfect sense....

The larger transfer port increased the power a bit, made the shot strings more curved (and hence shortened them), and required about 3 turns less preload on the RVA for the same FPE level.... Possibly 1 turn of that is likely due to the slight increase in hammer weight with the longer guide and the cocking knob.... Here are the results of the testing, from coil bind to 12 turns out, by which time the fill pressure required was less than 2000 psi....



You can see that with a 3000 psi fill, anything within 6 turns of coil bound, while producing a bit more power, drastically shortens the shot string, and of course wastes a lot of air.... I saw 1000 fps (102 FPE) with everything maxed out at 3000 psi, however.... but the gun ws using nearly 200 psi per shot, and was down 4% in just 3 shots.... The best setting I could find was 7 turns out, starting from 2900 psi and ending at 1900.... It peaked at 929 fps (88 FPE), and averaged 913 fps (85 FPE) over 11 shots.... but the efficiency was only 0.88 FPE/CI.... This is the first gun I have tried where the area of the transfer and barrel ports (0.219"D) were larger than the area of the throat, which is the equvalent of a 0.203" hole.... I'm pretty sure that is why I couldn't get over 1.0 FPE/CI like I did with the 3/16" transfer port in the initial trials yesterday.... This leaves me with two choices, either drill the throat out to 1/4" (it is currently 15/64").... or refit the 3/16" transfer port.... The former should increase the power even further, and may increase the efficiency slightly.... The latter will drop the power a bit, but increase the shot count and efficiency.... I will wait until I get the new longer barrel to make the final decision.... but since I'm already pushing these pellets into the mid 900s more power may not be the answer.... so I'm leaning towards going back to the 3/16" transfer port with the 25" barrel....

Bob

[Sqrl Klr]

Very nice Bob. I like the look of the handle. Lloyd and I have talked about switching to a rear cocking knob on mine which I may decide to do in the future but I would also have to modify the breech to mount the scope more forward so it would not be blocking the knob making it hard to reach. Is the scope a factor in your case too or can you reach the knob good?

[rsterne]

I just mounted a scope, and it's not too bad to cock at all.... actually much easier than a round knob.... You can sneak your index and middle fingers in flat under the scope, there is lots of room to hook them on the handle securely and pull straight back.... You may have to lift your cheek off the stock a bit depending on if you're a stock crawler.... I think it's far superior to a round knob, I've had those slip from my fingers and fire while cocking.... not the nicest of feelings....

Bob

[Sqrl Klr]

As would I for the same reason plus it must be much more comfortable as well where I could see getting knob rash from a round knob if using a thick spring at max adjusted power. I used to get bolt rash on my index finger when using the .047" spring with the bnm knob since it was so thin. I personally think yours just looks better than a round knob as well.

[rsterne]

In case anybody is interested.... I used a 3/4" mill and the notches are 1" apart C-T-C.... Total width is 1.6", depth is 0.4", material is 3/8" thick 6061-T6.... Everything else pretty much falls into place.... The cocking rod / spring guide is a piece of 7/32" drill rod (works perfectly with the QB spring) and it's turned down to 3/16" and threaded 10-32 on both ends.... with the hammer (and handle) drilled and tapped to match.... after installing the guide in the hammer (tight, with blue loctite), chuck it up in the lathe and make sure the rod runs true (turning it by hand).... If it doesn't, bump it with the crossfeed on the high point until it starts to bend.... Check it again, and repeat, turning the crossfeed in 0.010" further each time until it runs true.... The RVA screw (3/8-24 NF) is drilled 15/64" from the back end for most of it's length, with the inner 1/8" drilled with a #2 drill (0.221").... If that's too snug, use a #1 (0.228").... The guide must slide easliy through the RVA screw no matter what the orientation or stroke position.... If you're using this on a Disco spring, use 3/16" drill rod and a smaller hole in the RVA adjuster....

Bob

[Rdsail]

Bob,

Coming along nicelt. you are doing so amazing work. I bet is a very loud gun. Are you going to but a shroud on it once your barrel comes in?

[Bruce]

This is a very interesting Project! I bet it turns out awesome! Heck everything you do turns out amazing!

Thanks so much for all the info you post! I enjoy reading it so much!

Bruce