Let start off by saying I'm not trying to bust anyones b-lls here or whine like alot of gun geeks do on forums. Just trying to spitball ideas on a problem.
I've got one of your ap4 .308 rifles that I just got back from repair for failure to feed problems the bolt feels sluggish like something is stopping it. I was checking the mags if your looking at it from the rear the left side rear most rib catches on the space in in between rim of the round holding up forward motion, also the mouth of the case is getting hung up on the front lip of the mag. The barrel extension ram did needed an lower angle but I'm feeling that the mags are where the germlins are.