Supplies:

6 redstone repeaters.
About 12ish redstone
1 button
6 TNT (Per shot)
1 Bucket of water
1 Slab
25 of any block(s)

Steps:

1. Make 1 row of 8 blocks.

2. Put 1 block next to the end, then another row of eight

3. Put a slab at the end, making a box of 6 blocks

4. Empty the bucket at the end without the slab

5. Put a block over the source, for protection

6. Raise one of the rows to 2 blocks high

7. Put one redstone on the 2 block high row next to the slab, then 6 repeaters, ticked all the way back, facing the redstone on the blocks, with another redstone on the end

8. Put redstone on the 1 block high row, ending before the block next to the slab

9. Connect both redstone trails, then connect it to a button

10. Put TNT in the water (NOT on the source) and on the slab

11. Press the button: Fire in the hole!
9. Connect both