[Boyso]

You piston head is ported. When it moves forward, the o-ring expands tightening the seal. When it goes back, the o-ring retracts.

Using silicon oil of heavier weight on the cylinder walls and on the o-ring will reduce the friction.

But at the same time, my AEG with almost everything stock has poor airseal with the piston head/cylinder/cylinder head. Still, I'm getting 347 +/- 2.