M16/15 feeding problems, Cont'd

As you noted, bullets were hanging up on the feeding grooves.
These grooves (feed ramps)are machined into the barrel nut.They need the barrel assembly to be removed from the receiver to properly access and polish them. Like feed ramps in any other gun, polishing to remove lateral machine marks improves feeding and minimizes distortion of bullet tips.
I've used a Dremel and Cratex rubber bonded abrasive wheels to do these feed ramps and would recommend the same for serious AR shooters.