Waterdog, that's a problem with an easy solution. Check the VDI of the offending screw cap and reject those VDI's. It's a solution that I try to avoid because by doing that I loose Indian Head cents and other desirable targets. But at some sites it is the only solution that I have found. It boils down to "trash tolerance". If there is only the occasional screw cap, I grit my teeth and hope that it is a good target and dig. - you never know until it's dug. On the other hand, I remember a site, about a half of a city block where there was an old speak easy or after hours bar that had been torn down. On EVERY swing of the coil I had 1 to 3 screw cap targets or other junk. It was unhuntable using my normal wide-open discrimination settings. Knowing that it was in a potentially very productive site, I rejected from positive 74 down and just searched for wheat cents and silver.I also switched to the 4x6 coil. In the course of several hunts I dug over 20 mercs, a few Walking Liberty Halves , silver quarters, some silver jewelery, and plenty of wheat's. I hate to use that much discrimination but some sites dictate that strategy. Best of luck and...
Happy Hunting!
Blind Squirrel