Works very good

I use this app with an iPod touch and it works great under most conditions. It requires an external mic which doubles as speaker for the beep. It isnt very loud, but generally can be heard with hearing protection. The sensitivity can be adjusted to suit most firearms and conditions. It wont take the place of a dedicated shot timer, but for casual practice, it works great. Plus its free.

Barls44 about Gun Shot Timer, v2.1