Rusty bottlecap tid numbers are fairly unstable for me (this becomes more apparent audibly if you are using 50 tones. Also, if you do a 5 khz bottlecap check to see if ID changes or if you simply lift or slighly pump your coil, it happens) vs. actual silver. Also, I continuously hunt in all metal mostly and with all metal engaged, the rusty caps also give an iron grunt that is enhanced with higher iron bias settings (you don't need to hunt in AM though, you can just check the target as needed with the AM horseshoe button too). Also, yes you should ground balance, otherwise you will get a lot of ground noise in all metal which will make all non-ferrous appear to grunt. Unfortunately, really deep silver can start to get an unstable ID and grunt too but the high tone sounds more "solid" (advanced skill. this is pretty nuanced).
Nails are similar but have more of a ferrous grunt and don’t typically sound like coins because they are not round discs, therefore many of the above skills are applicable.
Yeah, I have to train my ear on each outing and may dig a few crown caps at the beginning, but after awhile their tells (unstable id, grunt, "hollow" high tone) become pretty obvious, especially if I am also digging silver. It is pretty nuanced, and a learned skill that takes practice. There are no magic settings that make this obvious without practice and a lot of regular swing time and the "skill" dissipates with extended time off between outings. Hope you can handle that truth, Truth.
Update: BTW as High VDI said below, if you are not digging iron, you are probably missing targets. One way I look at using the above tips, even if I am going to dig the iffy signal anyway, is setting my expectation appropriately that the target probably is junk, so if I do get surprised, it is a pleasant surprise. The key is recognizing you are digging an iffy signal in the first place.
HTH HH
