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New C9 consistently jams
Did you try to check/adjust the front feed lips of your mag? Do you have another mag to try?
The front lips might be too far apart and let the front of the next shell pop up that might be pushing up on ejecting shell jamming it.
Good observation on the nose up with less than 4 left. I've noticed that too. I think because of the taper of the shells, when to mag is full to about 4 left, the shells are pointed down below the front feed lips, I think the front feed lips help keep the nose down with all that spring tension from the first 6 .Then it looks like the rest will be above the the front lips but there is less spring tension on the mag and it is more of a direct feed in chamber.
Find the picture of the Mag with the distance specs and adjust accordingly. If you don't have a caliper, you can use a 9mm bullet at the bullet diameter as a gage it is .348 it should just slide through front lips.Not the shell part .If that passes through the front lips,its too wide.
Hope this helps.