Hopefully this answers some of your questions . . . and others will provide their experiences . . . .
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How good does the machine work in iron, i.e., cellar holes, camps, etc..?
I have had the X-50 and the X-70 to several old foundation sites and was able to pick out good targets from the iron. At times it was done by running with virtually no discrimination and working amongst the trash to pick out the high tones / squeaks that indicated a good target was in there somewhere.
My daughter was using the X-30 at a site dating back to the Rev. War and was in an area that while not carpeted with iron, had a good deal of it around from some old barn or the like that had been there. She got a clear signal that bounced on the target ID. Often this is an indication others have shown to be trash but since this site had age, she removed a few inches and pulled out a large wad of foil. Checking the hole, she got another signal and pulled out a rusted bolt. One final signal turned out to me an 1843 large cent at about 7". So even the X-30 seems have decent "iron see through"
The X-70 has a new feature called Target Stabilization which shows what can be done through software. Without going into a long discussion of what it does and how it does it (unless others want that info), it helped in high iron trash sites and bad ground to pull out goodies I would otherwise have left.
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Is the response faster than say the Explorer II?
Yes, both in terms of sweep and recovery speed . . . . . different circuitry and processor . . . . so faster speed but the FBS benefits are obviously the trade-off.
REMEMBER: IT IS NOT AN EXPLORER III!!!
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Also in the prospecting mode, there is an iron mask feature is that similar as well to the IM feature and can it be expanded?
No, IM in the Prospecting mode is simply a ferrous discriminator circuit - as it is increased from 0 to 20, some ferrous items are rejected but some small, low conductive targets; i.e., gold, will also possibly be rejected.
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Hope this helps . . let me know if you need more info
Andy Sabisch