TargetTalk

Has anyone tried a good shot detection app for iOS?

Look at TargetScan. Great for AP but I've not had so much success with .22 targets. Centre fire wadcutter targets should work too but the light behind the target is critical.

I meant shot detection as in audio detection. To display the time for each shot in rapid fire for instance.
Not scoring on paper targets

If it's for training purposes then an audio recording program & a copy of Audacity (or similar) should do it. Unless you're in need of microsecond timing.
Easier if you have an audio signal to time the first shot off, but the principle would be: load in the waveform, cursor over the spike that represents the shot & note the time. Repeat as needed.

In that case, try Shot Timer - its free.

Bob