Let's talk bottle caps with Racer. Here's the deal it's in the audio and the length of the audio. If you take a screw cap and any penny and lay them on the ground and sweep. You are going to notice 3 things. First the TID will be the same or close. The tone over the penny very clean vs the cap, and third the tone over the penny will not only be cleaner audio but shorter as well versus the cap. One exception: smashed caps will fool the TID and the audio. For the Racer owners/users out there. Try comparing say the penny and cap and see if you don't see and hear what I've talked about here. Many folks beginning with the Racers, it's understandable they wouldn't know the nuance of the machine with caps. The reason I have a head start is I've been running the Racer's big brother the Nokta CoRe unit. Both units are very close in performance and work the same so far for me over the bottle caps.