cleetus, I'd have to see the Fisher Gold Bug Pro do it, to believe it. The reason is this: About 10 yrs. ago, I was travelling through Los Banos, CA, and decided to stop and see the Fisher factory museum (since I like vintage detectors). After seeing their museum, I decided to ask the rep. there, at the desk, this very question: I asked him:
"Why, in this modern day and age of electronic marvels, that someone can't make a machine that can see through iron, like the old 77b could?" The attendant there fielding my question didn't know what I was talking about (he wasn't aware of earlier 1970's machines, etc...). So I invited him out to my truck, in the parking lot, to SHOW him what I was talking about. I set up about 3 nails over a coin. First I used a standard discriminator, and .... of course, it masked. Then I pulled out my 77b, and went over the same pile, and it gave a "beep". And just to make my point, I removed the coin, went over the pile again, and no beep! The Fisher rep. was intrigued, but quickly said "We have a machine that can do that", and went back in to the factory to get a Fisher Gold Bug pro. He came back out and tried and tried to get it to do it. But anytime he could adjust it to get a signal on the 3-nails-over-a-coin, then invariably, it would also get a signal off the 3 nails alone (with no coin). Turns out he was going on what he was told by others, that "it will do it". Yet he had never actually seen it or had need to do it, himself. Several more times we compared the 77b against his Fisher, on this parking lot test. And he came away seeing that his machine in no way compared to the 77b.
At least I think it was Gold Bug Pro he fetched that day. Is the Gold Bug Pro 10 or more years old? If so, then that was the one. And no, it couldn't do it.
Brian, the Groundhog's TR might have been "100kHz", but that was TR disc, not TR all-metal. And the disc. setting could not be reduced low enough down, in the factory preset calibrations, to do what the 77b could. In other words: yes, at the lowest settings it did knock out nails, of course, when in disc. mode. But at a high enough level that it also disc'd out whatever was underneath the nails too (ie.: masked). So that TR "disc" was different than the TR "all-metal".