[size=small]Pete (saika) said it best, having quickly become a TesoroMeister. But I'll try to add something meaningful to his already excellent comments.
Screwcaps in their pure form (undamaged), when presented to the coil vertically, will normally DISC out as we expect them to. That is, just below Cu cent range, somewhere along with those Stinkin Zincers. But they can be manufactured with variances and so may be off somewhat.
Too, they can be squished or mangled and then they can offer several possibilities to the DISC circuitry. They tend to overload when shallow, too, so your detector is masked by that, as well.
In fact, that is probably the best way to know thats what you are dealing with - by their size, when compared to concise, sharp coin signatures.
Too, a distinct possibility is that there could have been something else nearby (within 2-3") that was good and you were getting mixed results.
Did you catch the use of words like "tend to," "possibilites," "somewhat," and "variables." The point is, there are no absolutes, my friend (if you could produce THAT for us, your fortune would be assured!). But there are many, many variables and to do more than expect "good odds" in terms of precise ID is foolhardy.
The Tesoro line has always been one of the best DISC instruments out there. They are normally sharp-edged to the point of actually being reliable the majority of the time. I've used them for years and that hasn't changed. I am sure your basic Cibola is no different.
And that is what it is, a basic detector. I would like to suggest something radical here, sort of a "working with the detector, not against it," thing. Start training yourself away from twiddling that DISC knob all the time. Merely set it and forget it. Dial it to the notch below nickel and dig all signals that sound good when swept slowly, smoothly and methodically. Yes youll find lots of tabs and other crap - - - allright, twiddle it a far as pulltab range, but only now and then.
The dreaded screwcaps will soon give themselves away as being large and signalling long after a good coin dissappears. But dig em anyway -My most interesting gold find last year was a full row of front teeth, 18K! Large and heavy, I found it with my Vaquero, in a park littered with screwcaps.
Wanna guess where it DISC'ed out at when I tested it at the hole side? Dead in the Screwcap Zone.[/size]