Legendary artifacts can drop from engrams up to a max of 300. A more common location to find them would be tier 3 court of oryx. They are likely to drop there and there is the quest that makes you complete each level of the court that's end reward is a 300 artifact.
All rare and legendary engrams are capped at 300.
I infused that with the two rare artifacts from the last couple banners to make it 310 on my characters.
U also get a 310 from completing a quest for completing the raid if I'm not mistaken.
Outside of that, the raid challenge modes are the only way to get higher artifacts. Normal mode challenge gives a 310 artifact and hard mode challenge gives a 320.
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Thanks alot