[Schwag]

Damage is right. Sometimes the shimming isn't bad but the greases are awful and usually in big clumps. Gear grease gets in the cylinder and down onto the motor.
With my jg, I also changed the piston, piston head, spring guide, cylinder and inner barrel.
On my echo, the piston head was badly worn down one side, giving it a flat spot.
Still, if you're paying the u.s cost plus another 100-150 for parts, you have a great, powerful gun pretty cheap. Both my guns upgraded cost me less than a stock marui.