A simple command block contraption that will allow you to fire explosive arrows !
There's nothing to craft or to build, just active the command block
Click the blank sign on the side of the machine to destroy it (it was supposed to have text, but it bugs a little bit... Maybe it will be fixed in another version)
It uses a very simple tag system: when an arrow is detected as "in ground" (with the InGround tag), an IngroundCBC06 tag is added to the entity via a scoreboard, then each arrow with this custom tag will summon an explosion (PrimedTnt entity) and a lightningbolt where it is, and the arrow is killed to avoid unstoppable explosions.
Sorry for the (probably) wrong language, I'm not a native speaker :|
The command block contraption work in 1.9 and above