Cylinder & head question
 

I opened up a G&G GR300L mechbox, the O-ring on the ported head & cylinder fits loosely. Is this normal? There's ~0.5mm gap between the head and the cylinder.

/edit
Sorry, the O-ring barely touches the cylinder.

Thanks.

should be normal. the Oring is what makes the seal.

The O-ring barely touches the cylinder. Is this because the port holes force the O-ring to expand and seal the cylinder?

Yes, that's exactly why. If it wasn't so loose in the first place, there'd be a serious suction effect when the piston is pulled back at the start of the cycle.