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Old 03-26-2013, 06:57 PM   #1 (permalink)
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Hidden porting for classic Sheridan pumps

The porting is hidden unterneath the pump handle, in the join / chink between the tubes, in space where is the pump handle not guided, so the contact surface and pump-stroke is not affected. The change is hidden, is not visible from outside.







There is space for 2x 19 holes ID 2mm (see photo) or 2x 14 holes ID 2,5mm (see schematics below).

This way you can reduce the "sound patch" of your PGP. Subjectively the sound reduction is between 10-15% when you already have tunned marker with lightened hammer (I do not think that the porting will do something with the "Ploop" sound caused by the bouncing).

I was suprised, but the CO2 efficiency has been not affected, at least on tunned marker (the chrono results before and after do not lie).

Velocity graph of tunned marker after and before:



Schematics of the porting placement on ZGP-94:


Last edited by 3022; 03-26-2013 at 07:11 PM.
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Old 03-27-2013, 11:36 PM   #2 (permalink)
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I'd be interested in how it affects the sound signiture of the gun... Is the *plump* of the gun the same?
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Old 03-28-2013, 01:52 AM   #3 (permalink)
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That's a cool idea, but I think instead of putting slightly larger ports in the pump handle, I'd just have a groove runninging the length of the porting and opening at the rear of the handle, thus deflecting the gas discharge back at the shooter and away from who he is shooting at.
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Old 03-28-2013, 05:22 AM   #4 (permalink)
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Subjectively : -):
Untunned PGPs having classic "Ploop" sound, tunned PGPs after hammer lightening have sharper and shorter "Pop", hightly tunned long barreled scheridans have "Puf". With this PGP it has changed theo "Pop" to smother "Pluff". Sound signature is now somehere inbetween tunned longbarreled sheridan and tunned PGP without porting... closer to the PGP.

Along my experiences having groves alot better effect with decreasing of the pressure in the barrel, but they let the gas go out with far highter speed, so they do not limit the sound signature so good as larger nummer of small holes (when there is space for them).

I have avaited efficiency loss and from this reason I have used only small holes. But optimall will be combination - small holes in middle of the marker, below one side of pump handle and shorter groove on other side, near the muzzle. Perhaps ther last 3-4 holes should be conected in groove.

Great invention - thank you for inspiration!

Last edited by 3022; 03-28-2013 at 07:45 AM.
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Old 03-28-2013, 09:21 AM   #5 (permalink)
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Newbie question. What do you use to drill the holes? Drill press or hand drill?
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Old 03-28-2013, 09:50 AM   #6 (permalink)
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Quote:
Originally Posted by jellyghost View Post
Newbie question. What do you use to drill the holes? Drill press or hand drill?
Drill press. I will be not using common hand drill... but with Dremel and small sintered carbides milling cutter it should be perhaps O.K.

Biggest problem has been with cleaning of the exit edges / splinters from the drilling inside, especially in middle of the barrel. I have presed trought 17,4mm (11/16") ball bearing to separate them from the barrel and then very fine sandpaper (granularity 800) on wooden pencil to clean the finish. Then I have flex-honned the barrel again.

Last edited by 3022; 03-29-2013 at 08:25 AM.
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