Concentric coils especially the smaller ones don't seem to mask out with iron. I believe it is because dd coils have a very perpendicular signal and tend to reflect rather than consume targets. This would be why they go blind in one direction more than another.

Also try this theory on stacking quarters where most machines with dd coils will start to iron wrap at 2-4 quarters tall with the exception of low frequency of 5khz that is more sensitive to high conductors. A concentric won't iron wrap as it is able to see the target not only from above but the sides as well.

The Apex with the 9" detect will start to wrap at around 6-7 coils but will always produce a high tone and spike indicating a high conductor as far as 14 quarters deep but that is where I just stopped.

The likelyhood of seeing stacks of quarters in the wild is pretty remote but a small spill of 2-4 quarters might be possible or maybe a silver dollar or half. Other thing to consider is maybe a silver dollar on edge then iron wrap is an issue.

I do wish Garrett would make a small concentric for the Apex too, would do really well in the iron riddled river I hunt quite a bit. So far I haven't seen any concentric options for SMF machines, wonder if there is a technical issue that makes it difficult or impossible? Maybe there just isn't enough of a market for it?