I know of many command block contraptions that run of redstone clocks. My favorite however, its one that turns the snowballs into grenades:
Here is it basically:
Have a clock running as fast as you can get it. Then put two command blocks off of the same side of the clock. One of them have this:
/testfor @e[Type=Snowball]
This will check the world to see if there is in fact an Snowball in existence. It IS case sensitive as well.
Then have a comparator running off of the testfor command block that has this:
/summon PrimedTnt ~ ~20 ~ {Fuse:20}
This will effectively spawn a ready-to-blow Tnt 20 blocks above the Command Block.
Finally, off of a different branch of the clock, have another Command Block:
/tp @e[Type=PrimedTnt] @e[Type=Snowball]
This will teleport the TNT to the snowball before it explodes!!
Alternatively watch this video:
https://www.youtube.com/watch?v=le-UjFVDHhA
