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PostPosted: Mon Jan 26, 2015 5:00 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
Posts: 8973
Location: Coalmont BC
The last time I was working on my .25 cal Disco I replaced the barrel with a .25 ACP liner from TJ's to specialize it for cast bullets, and replaced the tapered MRod poppet with one made from PEEK.... This gun has a pretty fancy valve in it, using a 2-piece design as in the photo below.... The valve is secured with three high tensile 10-32 screws to allow safe fills to 3000 psi....

Image

The valve has full boresize porting and a 9/32" throat, the only restriction below 0.250" is the barrel port (an oblong port the equivalent of 0.240") and a slender probe on the bolt.... I recently made a 2900 psi inline regulator, so now it is easy to tether any gun at that pressure to check out how it responds to changes in hammer spring preload.... That way I can determine if I have enough power in the hammer to max. out the performance at 2900 psi.... I ran those tests today, and here are the results with three weights of cast bullets.... 0.5 Turns past flush on the adjuster was the maximum preload I can use and still cock the gun without coil bind on the QB hammer spring....

Image

As you can see, I have just enough hammer strike available to max. out the velocity of the gun, as shown by the plateaus on the left of the graph.... With the 58.8 gr. Mr. Hollowpoint bullets, the gun peaked at 960 fps (120 FPE), and achieved 115 FPE with my 50.6 gr. Bob's Boattails and 111 FPE with 46.4 gr. Hollowpoint version of the .25 ACP bullet.... These represent the range of bullet weights practical for this gun at this power level.... which is 5 times the FPE of a stock Disco....

I dropped the hammer spring preload back to 1 Turn out from flush, and the gun was shooting consistently at 960 fps (104 FPE) with my BBT's, at just under 1.0 FPE/CI efficiency.... Filling to 2900 psi and dropping the gun off the tether allows three shot of 960-966-956 (a 1% ES) before the velocity drops off.... That's a 3 shot average of 961 fps (104 FPE), and I can drop the gun back to about 97 FPE and get 5 good shots within a 4% ES.... From looking at it, you basically can't tell the gun isn't a stock Disco.... It does, however, have a bit of a bark!....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Mon Jan 26, 2015 6:45 pm 
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Joined: Sun Feb 10, 2013 1:16 pm
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Interesting design. Is that large hole in the valve a 1/8 npt for a gauge? I've wondered about doubling up the o-rings, but wasn't sure if it made sense or not. I guess it must do if you're doing it. For moving parts you'd have to weigh up the extra friction against the reduced chance of leaks. Tempting for the piston in my 1377 as it seems to be a bit hard on o-rings, although I have some new, hopefully higher quality ones, to try.

Jim


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PostPosted: Mon Jan 26, 2015 7:05 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
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Location: Coalmont BC
Actually the only reason I used two O-rings is that is the way they do the Disco.... I don't think it's necessary, or even desireable, as it can reduce the pressure differential across the O-ring and increase the chance of leak, so I have been told....

Yes, the 1/8" NPT hole is for the gauge, I have integrated the gauge port into the front half of the valve.... I have a simpler to make one drawn out, using a conical (tapered) spring and a single 3/8" hole through the front half instead of the four angled ports....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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