Thank you, glad you like it.
You got dud when both in the column and row there are :
1. two or more skull.
2. nothing match
For example in the highlighted row, you got Explosive-Skull-Skull. But in the highlighted column, you got Chain-Chain-Slow. Your bullet will produce Chain Lv1 effect. But if the highlighted column you got Chain-Skull-Skull or Chain-Explosive-Slow, your bullet will produce a dud.
I am aware that I need to clarify that and gives the effect of each slot, not just the skull. I just ran out of time to do that. I will focus more on this and the presentation in the next update