I do not know why the Equinox is so good at this? It is not only nails, but round targets as well. Where I hunt there are coins with a ferrous and non-ferrous mix. I can identify them at fringe depths 95% of the time.
It is not only the double beep, but also the breakdown of response. It is a two-prong approach. For those fringe targets that do not double ring, they might have a serious breakdown of response in a 360 degree look. Non-ferrous targets will NOT have a breakdown of signal response on a 360 degree look.