I've been running Coil repeatedly to craft all of the Undying Weapons. I've gotten every weapon crafted except Subjunctive. Everytime I've ran Coil, I've never seen an SMG as the end reward on every run. This has to be a Bug and not RNG, considering Subjunctive is a Legendary Weapon and not some Exotic Weapon from a Raid. Hoping a fix is in the works and will be patched soon.
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Running the Starcrossed mission seems to have a higher chance at dropping undying weapons. I know I've gotten what few red borders i needed from there more often than from coil