http://AirsoftTech.dk/Guides.cshtml?Page=BarrelStabilization_Page1
Allot of the jg g36 has a verry loos fit inner barrel ... So this may help ...
Also if you need a disassembly guide:
http://AirsoftTech.dk/Guides.cshtml?Page=JG608_Page1
And as for parts: Tight bore barrel, and some soft rubber (Eg promethius purpal) ...
And if you have changed the cylender or barrel considder dooing some calculations on cylinder to barrel ratio calculations.
This eksplains the principles:
http://AirsoftTech.dk/Guides.cshtml?Page=CylBarVolRatio_Page1
And this is a calculator you could use:
http://AirsoftTech.dk/Calculator.cshtml
Also: Based on my experiance with that speecific gun, the air sealinng is not verry good :-/ so you may increase the power a bit by air sealing ....
http://airsoft.spells.dk/Guides.cshtml?Page=AEGAirSeal_Page1
http://airsoft.spells.dk/Guides.cshtml?Page=ORingExpansion_Page1
Hope it helps :-)