Only time I've been around one it didn't work so well, but that was on a line of about 40 rifles and the guy that owned it was bitching up a storm that it wasn't working. But when you have that much ambient noise going on, it's no wonder it didn't work too well, not to mention the amount of lead flying through it's field of view.

That said, I love my magnetospeed, and I've never needed more than that. Beats the hell out of setting up a traditional chrono.