[stagpanther]

Do you have a spare magazine?

Using the 350L in a 5.56 AR magazine has similar issues, the way they are stacked--and the fact the 350 L is relatively straight-walled all the way to the bullet ogive, means that it is not going to feed the same as a 5.56; my guess is that sideways pressure out towards the nose is changing the feed and causing the nose-dive (unless your cartridges are too long). You may want to sacrifice one magazine by modifying the feed lips to see if you can get a reliable pitch up of the cartridge when pushed by the bolt.