Heroic patrols and the weekly mission are the best soruces. Its a 100% drop rate for a weapon from a heroic patrol but if its "red-border" is all RNG. Count your self lucky it use to be way worse. I mean its still bad but heroic patrols use to be about 1 in 10 however Bungie substantially buff'd they rate they can appear with season of the deep. Overload, public events, and heroic public events CAN NOT drop neomuna weapons that have a pattern. To give you some perspective on how bad the grind is. Twitch streamers would ask their viewers to find heroic patrols for them, then the streamer joins them. So how you like your chances at that red-border grind when some one with 100+ people more or less doing the grind for them still struggled to finish the neomuna weapon patterns?

heroic public events are the only way afaik