Local Part Magnitude?

I've tried using a local part to try to find the magnitude between a local player, but it never works. I'm not asking anyone specifically to give me a script, yet I just need to know where to start.

For example, in many fps games, whenever a bullet flies by you (very close), it makes a sound or creates an effect only you can hear/see. Though, like I said before, I only want to know where to start, and if this is even possible with a local part.