I thought I explained why, see, I'm a wizard, but the person shooting the arrows is a ranger. Sure, I could put a rune on the arrow, but then I would have to waste an action to watch when the arrow is about to hit, to make the rune go off. Not to mention I'm not even sure how you accurately portray something like that in terms of skill checks/ability rolls.
Explosive runes seems to be set off when read, I didn't think just calling out the key word would do it. Would it?
I thought a TNT equal or gunpowder usage was available in D&D. I just didn't see anything that put the two together.