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Discussion Starter #1
Ive got the Action army piston and it has been working brilliantly after sanding the rear of the piston. However after my last game it has just lost all compression, it was 100% perfect I could put my finger over the end of the barrel i got as far as waiting 2 minutes before releasing and it still fired as normal. But since this last game if i try this it doesnt work after even a second. Ive opened up the cylinder and worked out its definitely the piston/Oring as even without the cylinder head on by covering the entire thing with my hand it slides smoothly without any resistance all the way up to my hand. I have checked its not the hop up etc and it is definitely this causing the issue, Is it possible air is leaking behind the o ring as a result of the ports on the head? if so should i plug them?
 

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The ports are a one way valve, and nothing more.
They are there, to make sure you don't fill the cylinder(With air), by sucking air through the nozzle(Instead of the nozzle and the ports together), as this could result in increased resistance.

But in a BAR they are less important, as you rarely cock the bolt fast enough to experience this.

In a full auto AEG they are essential, as the cocking rate somewhere between 0.05sec(ROF 10) and 0.0083sec(ROF 60) in extreme cases.

The thing about better compression, it's something manufactures use as a selling point.
 

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Discussion Starter #4
I have swapped out 13 different o rings of varying size and it has made no difference so far, It just made the bolt return stiffer.
 

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Discussion Starter #6
the cylinder seems in standard shape, ive just cleaned it out and it is almost to a mirror finish (slightly murky mirror).
 

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Discussion Starter #8
It is an HFC cylinder, but hasnt caused me problems so far. I dropped in the stock piston, which works much better with the same o ring. Ive concluded that it must be the cut in the nylon ring which seems to be causing the issue.
 

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My guess would be the airbrake hole. Either the airbrake is missing or the replacement screw that comes with is missing.
 
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