I had a similar problem not to long ago... I figured out that I had several "bad" mags. What would happen is the bolt would push the round forward but the forward inside lip of the mag would catch the edge of the case and from the force of the bolt it actually pushed the bullet back into the casing. I think the springs were bad but didn't bother to change them. Try using several different mags when you go to retest it again. When you get a misfeed mark the mag with a piece of tape or something. continue cycling threw back through all the mags and see if you only have problems with certain mags and if you do get rid of them or try changing the springs.