Classic army AK making grinding noises on full auto
 

So I have had a classic army RPK for a few years now but haven't been able to use it. When firing full auto, it sounds like it's making a grinding noise every few shots and the bbs sort of roll out of the barrel, however it fires flawlessly on semi, and dry fires full auto no problem. I've tried cleaning the barrel, but other than that I'm not sure what to do. Any suggestions as to what this could be?

Did you replace the shitty yellow piston that CA has?

wow, have you tried other mags?

compression jamming... BBs are jamming due to misfeeds or not leaving the hopup chamber and the nozzle has nowhere to go... grinding noise is probably coming from preengagement happening since the air seal is greater than the resistance on the BBs being held in the chamber.

The piston is probably in some shit shape right now, maybe even the tappet fin. On top of that you have the sealing issue that's causing the jam. Could just be due to poor hopup rubber install or major air leak somewhere. Ever get it chronoed?

I have tried different mags, and when I had it chronied it seemed a bit low.