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Thread: Curiosity and the Proverbial Cat.

  1. #1
    Senior Member Doc Sharptail's Avatar
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    Curiosity and the Proverbial Cat.





    This has been gnawing at me for a while now- to the point of an "itch".

    First, a word from the safety dept:

    I don't recommend anyone doing this modification. The stock CO-2 cap is a bit on the short side for this application. The web at the bottom of the plug is just slightly over 1/8" thick. Definitely not to be used with high pressure. Anyone considering this should get either the brass bulk fill cap, or the factory bulk fill cap- both available from Eric at scopes and ammo.

    I had a bit of concern getting this set-up past the vent hole in the stock tube. Rightly so- it's gonna be a bear to get off...

    After all that- it works!

    I was very leery of that thin web at the bottom of the plug- enough so that I wrapped the rifle in heavy burlap while filling it. I let it sit in the burlap 20 minutes after filling, to make sure.

    Results that I wasn't quite expecting with such a small volume. The valve has not been touched on this 78-D. It still has it's piercing pin, and cigarette filter in it.

    850 PSI, with the 14.5 gr RWS Superdome:

    1.) 519.1
    2.) 513.6
    3.) 500.0
    4.) 481.8
    5.) 469.9
    6.) 440.8
    7.) 428.2
    8.) 413.4
    9.) 393.1
    10.) 374.9

    Test halted. I didn't want to burp a superdome so it was stuck in the bore.

    1500 PSI with the same pellet:

    1.) 654.9
    2.) 644.8
    3.) 634.7
    4.) 635.5
    5.) 629.2
    6.) 624.1
    7.) 615.4
    8.) 611.8
    9.) 607.8
    10.) 599.9
    11.) 588.9
    12.) 582.1
    13.) 572.4
    14.) 557.3
    15.) 544.9
    16.) 531.6
    17.) 514.6
    18.) 499.7
    19.) 483.0
    20.) 472.8
    21.) 446.5
    22.) 431.7
    23.) 414.2
    24.) 400.4

    With either/or/and a moderate hammer spring clip/port restrictor, I'd have me a fair to middling indoor sub pal plinker in .22 with this. I'd expect even better in .177, which I'll get around to, sooner or later.

    A few important points to cover:

    The stock tube on the 78-D is heavy on the inside with machine and tool marks. I still think the 3K tube is the way to go. I had a bit of apprehension taking that stock tube to 1500. FWIW, the inside of the stock AR-20-78-A tube is much cleaner- almost as if extra attention was paid to it...

    I'll be discarding that modified tube cap. I don't have that much trust in that kind of steel- it crumbled a bit under the drill, and under the NPT tap. I think for what it was made for- 2 CO-2 caplets- it's fine. Anything else is sort of "asking for it".

    Regards,

    Doc Sharptail
    "Ain't No Half Way"

    -S.R.V.

  2. #2
    Moderator rsterne's Avatar
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    Interesting results.... Here is how I did the Fill Fitting (top right).... All the cap does is take the end force from the brass plug....



    What I did find interesting with your attempt is that you didn't get a curve starting at 1500 psi.... Here are my results....



    The lower blue line was with a stock 78-D with the only mod being drilling the bolt probe out to 5/32".... It started at 1600 psi, peaked at just over 600 fps at about 1200 psi, and ended at 600 psi, with all shots within 10% (as befitting a short range plinker).... You obviousy have a better hammer spring than my gun did.... You are correct that clipping the spring (to get a curve) and restricting the port (to drop below 500 fps) would be a great non-PAL plinker....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  3. #3
    Senior Member Doc Sharptail's Avatar
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    Bob-

    If I'm judging the scale of your photo correctly, I may have a slight volume advantage- there's a 1/2-3/4" difference in the length of our plugs (get lost Freud )

    As to the hammer- it was cleaned and diamond clothed, and anhydrous graphited. The trigger parts on this gun have a high degree of polishing.

    Different guns, different gauges, different pellets- it's pretty well a given results are gonna be different.

    All that aside, I'm pretty sure I would have gotten a curve with one, or two hundred more PSI. We'll see when I get a safer fill fitting in. In the mean-time, I'm going to put the 20-78 tube on that gun...

    Now to put a parts order list together...

    Regards,

    Doc Sharptail
    "Ain't No Half Way"

    -S.R.V.

  4. #4
    Moderator rsterne's Avatar
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    I have the O-ring inside the thin threaded area of the tube where it's full wall thickness.... Having said that, I could have probably machined off the part where I had the flats for the wrench and if necessary put the flats between the O-ring and the outer end.... Mine was only for temporary testing and was replaced by a tank on a tube that was half way between a 78 and 79 tube in length....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  5. #5
    Senior Member Doc Sharptail's Avatar
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    Finally got my order off to Eric

    I can't quite figure the anticipation. It has a strange affect on me. One of the things I ordered was a .177 bolt for the A/R bbl I ended up with in trade. That anticipation leads to odd behaviour- I installed the .177 bbl onto the 78-D- even though I'll have to tear down again for the valve stem replacement. I did get the tubes switched, though. I'll probably end up shooting it on CO-2 first, just to see... that .177 anticipation is as bad as waiting for a new rifle

    Regards,

    Doc Sharptail
    "Ain't No Half Way"

    -S.R.V.

  6. #6
    Senior Member Doc Sharptail's Avatar
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    Quote Originally Posted by rsterne View Post
    What I did find interesting with your attempt is that you didn't get a curve starting at 1500 psi....

    Bob
    Sir, you are correct again! Had at the valve, and found a factory mis-assembled interior. The stem return spring was wound half way through the washer, and the cigarette filter totally shredded. I'm amazed this gun held even CO-2, let alone 1500 PSI of air...

    Regards,

    Doc Sharptail
    "Ain't No Half Way"

    -S.R.V.

  7. #7
    Moderator rsterne's Avatar
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    I assume you threw out the filter and screen.... they are just a PITA....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  8. #8
    Senior Member Doc Sharptail's Avatar
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    The only screen I've ever seen was in the replacement valve I ordered from Eric. Both rifles had none. That filter pocket may come in handy for volume reduction. Haven't decided yet...

    Regards,

    Doc Sharptail
    "Ain't No Half Way"

    -S.R.V.

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