The best coil is the 10x12 inch concentric coil.
On a new Silver uMax it will find coins down to 9 or 10 inches which puts it in a class with the Explorer SE, GT, etc., plus the Tesoro Tejon, etc., etc.
You need to slow down and remember it has a cone shaped signal. At extreme depth you're only covering an area of about 4-5 inches in diameter. So overlapping is a must.