Mine does this with the US mags from HKparts. I ended up swapping springs, followers etc and found that the problem stayed with the particular mag bodies. I also found that when left loaded and not in the gun that the feed lips would widen slightly. I think it just comes down to the US made mag bodies don't have the reinforcement or something in the feed lips and they let the round out too easily. I have never had a jam with the HK original mags.