I don't know about anyone else, but I'm sitting on tons of weapons I crafted, and I have zero time or interest in leveling them up to 13 or 16 to actually make the gun I want to make. It would be great if you could use a certain amount of shards or planetary materials to gain levels in addition to just using the weapons.
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Or just allow kills to give 1% per red health add. 5% per major. 10% per ultra/boss. And make XP also level weapons. 1% every 500xp.