Many might have noticed the influx of threads about rigged RNG and not getting your weapon in X boxes despite the chance being as it is, all that jazz.
I have compiled a helpful chart to shed some light on the situation.
Now as you can see here, because of the numbers of boxes getting quite large (Running a dungeon 20 times is quite a lot!), the amount of threads starts rising early into the blue curve, representing the chance of still not having your weapon. I assure you, however