It has been demonstrated numerous times that the FBS detectors have poor reactivity in the presence of iron. Yes, in the real world it can find silver amongst iron, depending upon how the iron bends (deforms) the magnetic field. But all things being equal the FBS detectors do best at maintaining depth and target I.D. in the presence of magnetic soils (to include iron fields), that's their strength. Other detectors in light-med magnetic soils will be deeper, with much better target separation (quicker reset), and much better reactivity to non-ferrous targets in fields of iron. Increase the density of the magnetite matrix and the FBS detectors begin to nudge past their rivals - as far as depth and target I.D. goes - which can yield the silver in bad soils.
The E-Trac and CTX are both dual freq detectors with the primary freq being very low, which makes it dead to iron (and other low conductors, as iron has high resistivity and needs higher freqs to energize the eddies). But in uniform iron, the machine can spark (high tone) on those higher conductors, especially in heavily mineralized soil. Unfortunately, it also makes it nearly dead to small jewelry and other low conductors (small, thin coins). A 90% silver 1775 1/2 Real near iron nails can be missed due to being co-located with iron under the coil, with the E-Trac and CTX. Once that is recognized, the best mitigation tactic is to attack from various sweep angles and go slow.
