The most accurate method to start the clock in SprintTimer is to use the sound from the starting gun, either directly in Photo Finish or via Start Sender. You need a microphone that is placed close to the starting gun to get a sound that is significantly louder than the surrounding noise (cheering crowds, wind etc). But if you have an electronic starting gun an even better solution is to connect the starting gun directly to the headset jack on the iPhone/iPad.
The main advantage with a direct connection is that it is completely insensitive to external noise since it is using the electronic signal, not the sound. Most electronic starting guns have a sound output port so they can connect to a loudspeaker. Since you still want the loudspeaker connected you must first split that signal. The starting guns I have seen have a standard 3.5 mm 3-pole plug (TRS), so you can use the standard sound cable splitters that are pretty common and easy to find.
The second item you need is an adapter that connects the loudspeaker cable to the headset port on the iPhone/iPad. You can NOT plug it in directly since the connectors on the plug will be wrong. But there are a few converters available on the market. They are two types, one that assumes that the signal is pretty weak, e.g. coming from a microphone without an amplifier. The other assumes that the signal comes from an amplifier (line in) and therefore contains some electronics to reduce the signal in order not to overload the microphone port on the iPhone. Below are adapters from Røde och Headset Buddy.
I tested the set up with a starting gun from Jex Sport. It is sold under the Jex name, but also as Gill in the US and Hart in Australia. The gun has two ports, one for sound out, the other for connecting timing equipment. I only used the sound port (the timing port cannot be used with SprintTimer). Since the sound signal not is amplified, an iPhone mic adapter without any reduction is the best solution I tested both Røde SC4 and Headset Buddy ECM with good results. When plugged into the iPhone the gun gave a signal that almost hit the max on the scale, so I could set the start level to essentially any level to trigger a start. (Click on the image to make it larger).