Lex Augusteijn‘s high speed photographs are taken at the (relatively) glacially slow shutter speed of 1/10th of a second. By rights the bullet shouldn’t even be in the frame. He controls the camera, flash, and gun with a laptop, giving him great results with a relatively slow speed setup.
Lex described the process on Digital Photography Review:
Drop release, shutter, flash and coil gun all controlled by a laptop. As you can imagine, this requires very precise timing (up to 1/1000 sec) for all events. Flash duration 1/64 using Sigma 500 DG Super flash, this is approx. 1/10000 of a second.
(The above description applies to the water drop pic, not sure if the water balloon was shot — ha! — the same way.)