Got out to a park I've pounded hard in the past where I found an 1870s harness buckle guard and a few other things which were all deep and at the edge of detection range. Dug LOTS of holes today and got duped by many large bent nails and wire coils.
But I got 1 really beautiful consistent deep sounding signal. Took a while to get down to depth, but at about 11 inches I found a neat old pocket knife!