.30 cal Disco Double

[rsterne]

I think Sean is pretty easy to bribe.... *grin*....

Bob

[rsterne]

I got most of the remaining parts made today.... Here is the front breech mounting band, shown installed on the breech....



It fits over the tube between the trigger and the gauge, and securely clamps the front of the breech to the main tube.... I then made a new hammer and RVA assembly....



The hammer is the same length as a Disco / 22XX hammer, but it is cylindrical and will therefore work with a Disco, MRod, or PRod trigger group.... If an MRod trigger is used, it should be shimmed down 0.030" because it is designed for a thicker main tube.... The hammer is drilled to accept a QB 7X hammer spring, which is longer and larger in diameter, stiffer, and has more travel.... The cocking pin is moved back 0.065", allowing slightly more total hammer travel.... The hammer weighs 58 gr. the same as a Disco hammer, and has a notch to clear the rear breech screw location used on a Disco.... The spring guide is made from a piece of 7/32" drill rod with a nut threaded on, peened, and turned.... It is long enough to protrude through the RVA adjusting screw and serve as a cocking indicator.... It weighs 15 gr., so the total hammer weight with guide is 73 gr., 26% heavier than a Disco hammer....



You will notice that the hammer face is counterbored 0.070" to increase the hammer stroke and limit the valve lift.... In combination with the 0.070" shortened valve stem, the stroke has increased from 0.58" to 0.72" (an increase of 24%), and there is still 0.18" of valve lift available.... I have found that to be more lift than required on all other Disco platforms I have worked on, in fact it is seldom more than 0.12" in a properly tuned gun.... The RVA is made from aluminum, and carries an adjusting screw made from a 3/8"-24 NF bolt.... The spring and guide can be removed through the threaded hole, no need to remove the end plug once installed.... The guide slides easily through the adjusting screw, and protrudes when the gun is cocked, giving a visible cocking indicator.... There are two flats on the adjuster to allow it to be turned with a 5/16" wrench.... and it is locked in place with a thin (jam) nut once adjusted.... I have enough travel on the RVA to go from no preload to coil bind, about 16 turns....

I am now at the point where I can install the breech on the main tube and inlet the stock for the front breech band.... Once that is done, I can get on with sanding and finishing the stock.... Most of the gun should be done by the time the barrels show up....

Bob

[sholo]

...Breech and components look absolutely marvelous, Bob! Best design I think I have ever seen...

I am eagerly awaiting the day I have one of these bad boys in my possession!


Todd

[SeanMP]

Hmmmmm 971fps eh........see I knew calculating twist at a 1000fps was the right thing to do.

Bravo congrats That is simply epic.

[Doc Sharptail]

Looks good ,Bob! Can't wait for final everything to be done, and your results and opinions...

Now, some body come up up with a dual tube system for AR/QB :P

Regards,

Doc Sharptail

[SeanMP]

Looks good ,Bob! Can't wait for final everything to be done, and your results and opinions...

Now, some body come up up with a dual tube system for AR/QB :P

Regards,

Doc Sharptail

Absolutely.....Someday

[rsterne]

We had a slow week at the Motel, so I got a chance to get into the shop and machine the O-ring groove and port into the new barrel today.... I shortened the shroud so that when the air stripper is tightened the muzzle is just clear of the front of the threaded section of the stripper and got to take a look at the new gun assembled for the first time.... It still needs some parts blued, but here is the first photo with the new barrel, shroud and stripper in place.... The gun weighs exactly 8 lbs. and I'm extremely pleased with the appearance.....



The first shot over the Chrony was 1045 fps, but my tank need topping up, so after 3 hours on the ShoeBox, after dinner I got to shoot a set of strings.... I filled to 3000 psi and shot until the velocity dropped 4% below the peak in each case.... The last 4 strings required less than a 3000 psi fill, as I was reducing the hammer spring preload for each subsequent string.... Here are the results....



Maxed out, the first shot was the fastest at 1049 fps (112 FPE).... At 4 turns out, I got 10 shots that were within 4% ES, and they averaged 975 fps (97 FPE), peaking at just over 100 FPE.... The efficiency was 0.91 FPE/CI, filling to 2900 and refilling at 1880.... At 7 turns out, I got 12 shots averaging 912 fps (85 FPE), with an efficiency of 1.11 FPE/CI.... The fill pressure was 2400, refilling at 1520.... I think if I installed a slightly smaller transfer port I could stretch the string to 14 shots (2 clips)....

I'm very pleased with these initial results.... With the first serious testing of the new barrels producing MOA acccuracy out to 100 yards, this should be a serious hunter.... I'm looking forward to getting the new 7 shot magazines from Lloyd at AirGunLab.... He has converted .25 cal MRod magazine to .30 cal, and they look gorgeous.... My breech was made to fit them....

Bob

[sholo]

I'm extremely pleased with the appearance.....

As you should be...that is a gorgeous, well executed build, Bob. Awesome power, too - one of your finest builds yet IMO!

That one-magazine, 7-shot string is beautiful...last shot pretty well mirror's the first, and all 7 shots within 2.6% of peak FPS?......Niiiiiiiiice!


Cheers,
Todd

[rsterne]

I haven't tried this on a regulated PCP yet, but I would think you could set the hammer spring preload up for the knee of the curve, so that you aren't on the velocity plateau and therefore aren't wasting air.... and then choke up on the airflow by moving the nose of the bolt over the barrel port to reduce the velocity.... You have to have a probeless bolt design, sliding though an O-ring in the barrel (between the barrel port and breech), and a "J" slot which allows you to withdraw the bolt clear of the barrel port to start with, of course.... As with restricting velocity by using a smaller transfer port (or as the MRod does with a screw in the valve port) it seems to have little effect on the fill and refill pressures over quite a wide range of velocities, so no reason it shouldn't be just as good on a regulated gun.... The efficiency (in FPE/CI) doesn't increase much, however, it just creeps up as you drop the power.... Your shot count per fill increases of course, because the FPE (and therefore the air use) per shot is dropping....

The different efficiencies for the three strings I came up with have more to do with being in that part of the power band where things change quickly, and the dynamics of those tunes.... If you dropped way down in power to, say, 750 fps you wouldn't gain much efficiency.... just shot count....

Bob

[rsterne]

I got my new barrels today from Sean, and all I can say is WOW !!!



He machined the chambers, crowned and choked one of them and taper lapped the bore (1000 strokes), and then even polished the outside.... It looks like Chrome inside and out, I can't wait to see how it shoots.... The JSB .30 cal pellets are a PERFECT fit in the chamber and engage the leade beautifully, leaving good rifling marks on the head, but without smearing the area between....

In addition, he shipped me some of the CF Shrouds, and even cut them to length.... I had previously machined a Hatsan air stripper to fit inside the end of the shroud and made a .30 cal insert for it.... and it slid inside perfectly.... The shroud is 7/8" OD, to match the Disco and Disco Double tubes.... It will end up in compression, with the barrel in tension, when the stripper is snugged into place on the threaded muzzle....

Now to find some time to machine in the O-ring groove in the breech and the barrel port and thread the muzzle for the stripper....

Bob