GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: rsterne on January 26, 2015, 05:53:25 PM
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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....
(http://i378.photobucket.com/albums/oo221/rsterne/Discovery/IMG_2907_zps9a638591.jpg) (http://s378.photobucket.com/user/rsterne/media/Discovery/IMG_2907_zps9a638591.jpg.html)
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....
(http://i378.photobucket.com/albums/oo221/rsterne/Discovery/25%20Disco%20PEEK%20Hammer%20Preload_zpsmnwyfta3_1.jpg) (http://s378.photobucket.com/user/rsterne/media/Discovery/25%20Disco%20PEEK%20Hammer%20Preload_zpsmnwyfta3_1.jpg.html)
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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120 fpe is a lot found this stumbling on some old files. Always learning from you Mr. Bob
Cheers
Jay
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Funny you should bring this post back to the top today.... I had the gun down from its perch on the wall to change the barrel band to the "six-screw" version to allow the barrel to be supported, but still move independently from the reservoir with pressure changes.... This modification generally gets rid of any wandering POI as the reservoir pressure changes.... There are three 8-32 setscrews in the lower portion of the stock Disco barrel band, tightened against the reservoir.... In the top section, there are another three 8-32 setscrews to hold the barrel in place.... They are installed with blue Loctite, and tightened until they JUST touch the barrel, but with no tension....
The reason for dragging out this rifle was to ready it for testing the new BBT's that I should have molds for shortly.... The molds for the 41 gr. and 51 gr. .250 cal BBT's from NOE will be ready soon.... The plan is to test them against the 51 gr. BBT from LBT molds, and the .25 ACP ammo as well....
Bob
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Just curious, what is the barrel length?
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24"....
Bob
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Good work Bob. Im learning something...just not sure what :P
All seriousness lots of energy...lots of power and im quite sure its rather loud?
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VERY!.... especially if you dial it right up to maximum on the tether.... If you back it off for a 5-shot string not quite so bad.... *LOL*....
Bob
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24"....
Bob
Thanks Bob!! I was thinking it was 24".