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Thread: Regulated SPA P12 mods.

  1. #1
    REP: Froggy Custom Silencers

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    Default Regulated SPA P12 mods.

    Couldn't resist buying one when they were on sale for R3500 .... Figuring it would be a great project rifle.
    Initial impressions were positive... Build quality is good, machining and materials used are top notch.
    I quickly realised there were some issues when testing started.
    Rifle was extremely loud for the velocity... 855fps with JSB 18.1 gr.... It was an absolute air hogg...only 18 shots from 200 to 150 bar... Accuracy was terrible about 5cm groups at 20m.
    First mod was to manufacture a hammer debounce device. A softer hammer spring caused excessive hammer bounce.
    I used Sustadur PET plastic for the parts instead of steel too keep the moving mass as low as possible.
    6mm less preload than factory settings.




    A lot quieter!.... There's way to much hammer strike with factory setup for a regulated rifle.
    Velocity still 855fps fps, but shot count now 30 from 200 to 150 bar.
    I was puzzled why the reg was set at 150 bar, removed it and measured the plenum volume... Only 10cc! Way to small for 30fpe.
    The only way to property set up this rifle with the reg for 30 fpe and lower reg pressure +-120 bar will be to increase plenum to 20 or more cc.

    I'm going to replace the barrel, but wanted to see what improvement I could make to the standard barrel.
    The leade was extremely tight...
    Sizing the pellets excessively... After the first 3cm the pellets would fall freely through the barrel, only stopped by a slight choke at the muzzle.
    I removed the tight section.

    Shroud was slightly off-centre... Re-faced the threaded attachment while secured on the barrel.

    Accuracy is now acceptable. Velocity dropped to 845 fps... I turned velocity down about 10 fps from max before shooting the string.

    Groups shot at 36m... Left is 15 shots.


    There's regulator creep present... If the rifle is left fully charged for a few hours the first shot will be drastically slower.
    I inspected the parts under a microscope.... The white valve discs is badly machined and probably leaks where it seals against the set screw.
    I'll consider rebuilding the reg and making a larger plenum, or just remove the regulator.
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  2. #2
    REP: Froggy Custom Silencers

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    Still trying to find out if it's safe to fill to 250bar... No markings on the rifle.... Gauge shows 220 bar in the green.... Spa website shows 200 bar fill... Pellet gun said 250bar... Instruction manual shows 250bar for regulator installed in P10, I think the air tubes are the same.
    In the pic you can see the P10 has a much longer valve and larger plenum.
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  3. #3
    REP: Hill Air Rifle Pumps
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    Fill till 200bar.
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  4. #4
    Sharp Shooter
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    with a little work and a decent barrel...magic happens...10 shot groups at 25m



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  5. #5
    REP: Froggy Custom Silencers

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    Thanks Air Wolf! Then I'm going to remove the regulator.

    Dallas, very good result! I'll definitely be changing the barrel.... 16 gr JSB pellets spiral like crazy with the factory barrel at 890fps, only 18 gr shoot relatively accurately. The platform is solid and the trigger with softer springs and some fiddling is actually very good.
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  6. #6
    REP: Froggy Custom Silencers

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    Seems I had a brain fade moment.... Save working pressure is 25 mpa. Don't know how I missed that!
    I think 200 bar for the unregulated p12 is just to prevent valve lock.
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  7. #7
    REP: Froggy Custom Silencers

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    Decided to scrap the regulator.
    Made a new restricting valve stem with redesigned poppet and lighter valve spring.
    Black poppet is the new unit.

    Also added a bottle brush de-pinger... Works really well... actually less ping than with the reg installed.
    Happy with the shot string. I could have tuned for a flatter string but I want the ability to turn the power up to 45 fpe with JSB monsters.

    48 shots with es of 24 fps is not too shabby.
    Accuracy is terrible at the higher velocity... Pellets start spiralling above 850 fps with the factory barrel.
    I'll be installing a 60 cm Sumatra barrel next.
    Modified power adjuster, can now be turned with a small allen key.
    Last edited by Froggy; 11-03-18 at 16:02.
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  8. #8
    Sharp Shooter

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    Hi Ruan
    Very interesting modification, may I ask what the purpose is of the restriction on the valve you made?

    P.
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  9. #9
    REP: Froggy Custom Silencers

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    Valve throat restrictor works similar to smaller transfer port, but it reduces air flow at the source, not further down the line.
    Valve restriction gives better results especially if there's no o ring on the valve stem to stop air leaking from the valve towards the hammer.
    On P 12 valve throat is 6 mm and my restrictor is 5 mm....works out to 8.64 square mm...same as a 3.32mm TP.
    Last edited by Froggy; 12-03-18 at 10:42.
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  10. #10
    REP: Froggy Custom Silencers

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    Machined 0.5mm bushings because the Sumatra barrel is 14mm and factory barrel is 15 mm.

    Still need to make a new shroud... Just using my Sumatra silencer for now



    Barrel looks ridiculously thin without a shroud.

    Rifle should be tuneable too 60+fpe with the 24inch barrel.
    Last edited by Froggy; 16-03-18 at 13:05.
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  11. #11
    REP: Froggy Custom Silencers

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    Had to check velocity and accuracy.
    Same settings as short barrel.... Velocity increased about 40fps for 18.1 gr jsb's


    11 shot group... All the shots over the chrony...except the first shot on the left was the aim point for the group.
    20 m ft deadmans position.


    Turned the hammer spring tension up 2 turns for 25 gr JSB monsters.... 895...896...896...900...902...900.
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  12. #12
    REP: Froggy Custom Silencers

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    38m...5shot groups 18 gr jsb's...+- 930fps...middle top group is 16gr at +-990 fps.
    I just love Sumatra barrels!
    Still need to test 25 gr...
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  13. #13
    Sharp Shooter
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    can't ask for much more than that....nice
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  14. #14
    Sharp Shooter

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    Wow, thanks
    I bought 2 of these, one to stay standard, second for mods
    Looking at calibre change
    lots of homework at the moment
    Where did you source your new Samatra barrel if I may ask
    Cheers
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  15. #15
    REP: Froggy Custom Silencers

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    Hopefully I'll be able to shoot some 100m groups soon.... I'm very happy with the performance now! At first I wanted to shorten the sumatra barrel.... But the way it's performing now I'm not touching it.

    Barrel supplier is Pellet Gun
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