Turn 5 and last turn of coils.

[AzraelX]

I have yet to complete turn 5 but is it true that everyone that completes turn 5 gets a weapon. Is that true for every final turn of Bahamut?

[Totobi]

No.

/10char

[PandaPause]

Turn 5,9 and 13 drop weapons. However it is only 1 weapon per clear and its RNG based on which drops.

[Risvertasashi]

I have yet to complete turn 5 but is it true that everyone that completes turn 5 gets a weapon. Is that true for every final turn of Bahamut?

It is always the last turn that has weapon drops. But it's just as much RNG as any other loot drop.

I want my Dreadwyrm Cane ; ;

[Niqote]

The chests from coil work the same as the Chests from other content such as Primals and Crystal tower. You receive an item from the chest from a pool of possible items.
Its all up to luck if the one you want drops or not.

Here is a loot list in a nice convenient spreadsheet