G&G G2000 misfeed and Piston O-ring question

[Reaver_RRTS]

Bought my G&G F2000 (aka F2k) second hand from a guy who liked his Tar21 more. It has a few misfeed issues, but when I took the gun apart last night (LOVE the quick change system!) I noticed the piston had no resistance when pushed into the buffer tube. The o-ring seemed to be too thin to provide pressure against the piston head and the buffer tube wall to create a suitable seal. Would this affect the nozzle from retracting fully? That seems to be the main issue with the misfeed. I'm planning to change the o-ring anyway to something thicker, but will have to wait till tomorrow to visit fastenal for a replacement (if they have any, this is the first time I change o-rings normally I go to a retail store for paintball/airsoft but I dont have the time to go down to flagswipe in london for something they may not have.)

suggestions as to causes of misfeeding on the F2k? i am aware of the metal plate on the side of the GB it's already been filed down, and I noticed its clipping the white plastic spike opposite of it, so i pushed the block forward and screwed it in place so its just barely nicking it... best i could do.

Help would be appreciated, the F2K is my favourite bullpup rifle and I'm extremely excited to own it, just want to get it all fixed up for the 18th.