When the weapon parts were added to the Gunsmith's inventory they removed the ability to purchase ammo synthesis' for your primary weapon- you can still collect them, from chests on patrol/raid, packages from Variks, but you can no longer purchase them. I do not remember the patch notes suggesting their removal as an item for glimmer. Is this intended?
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Since almost no people use that type of synthesis and people wanted Weapon Parts more, Bungie just traded.