Simply spend a few hours playing prison of elders on the 28 difficulty. Pop special synth, every five minutes. You won't collect as many chests, but you will free up a lot of inventory space. And at the end there are two chests that can contain treasure keys.
It's easy enough that you can run with shotguns for most of it. The heavy you need it will give you.
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#Destiny
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Except at the end, in one of the small chests, I got spec ammo synth! Ef