"Buy weapons from Banshee-44" - Quest

["LivinG DeAd Youth"]

I'm trying to grind the Guardian Ranks, but the last Mission for Rank 4 won't complete. I've bought 6 different Guns, yet nothing happens. Is there any hidden Menu I'm missing?

[EckyThump]

If you're focusing engrams, that isn't what it wants. Banshee's inventory page should have a vertical button to move to a second page, with some directly purchasable weapons. Try those, if you haven't already.