I'm only about 150 rounds in on my 223 t3x and my 4rd factory mag seems to be having these same issues. It ran through the 150ish factory rounds without issue but I've just put together some handloads with old brass that I didn't tumble and it doesn't seem to like it. The brass isn't super dirty, it's just not factory fresh. It's the second round (3 still in the mag) that just doesn't quite move up into place so the bolt glides right over the top of it without touching. My AR never had any hiccups on dirtier brass than this so I didn't expect any problems.
I'll be tumbling my brass in the future so hopefully that extra slickness makes the difference, and I'll probably stretch the spring as Form mentioned, but is there anything else I can do to slicken up the inside of the mag body? The round just doesn't move to the center, it stays underneath the left side feed lip.
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