AEG - Air Seal
An airsoft gun as the word implies use air to shoot. That means that if the gun has leaks then it don't shoot as hard at it can. But achieving a perfect air seal can be very difficult. This guide focus on how and where to seal the gun to perform optimal.
Cylinder - Head
The first place to make air seal is between the cylinder and the cylinder head. This connection is often one of the best sealed areas in factory standard guns. So most guns don't need air seal fixes here.
To test the air seal you submerge the piston (with a finger on the end of the head) under water with air trapped in the cylinder. Then slide the piston in and put pressure on the trapped air. If bubbles appear around the edge then you need air sealing. If bubbles occur around the brass end then you have a cracked cylinder head that needs replacing.
To seal you can use Teflon tape and wrap it around the cylinder head. Alternatively you could also use RTV silicone sealant.
Piston - Cylender
When inside the gearbox also make sure you have good seal between the piston and cylender. The O-Ring Expansion mod shoul help you here.
Head - Nozzle
The second place to ensure good sealant in between the “cylinder head brass pipe”, and the air nozzle that move back and forward. Here it's not really possible to mod anything, but allot of cheaper guns have bad air seal.
To test the seal put the air nozzle on (Just a ½ centimeter like it would be in the gun just before firing) and close it with your finger. Then blow in the cylinder (with your mouth) or put pressure on with the piston. A good seal will have very little (optimal no) air flowing out. But a perfect fit would also restrict movement and it has to be possible to move.
To fix bad air seal you can for very little money get a air seal nozzle and that’s the best way to go.