I have a Garrett GTA 500 I bought about 12 years ago. Works good as far as i.d. ing the item and the depth, but I was wondering if maybe by changing the coil, I could get more depth? About the deepest it will go on the scale is about 4 - 5". I have the 8.5" Cross fire coil on it (that's what came on it when new). Does a larger coil go deepper and a smaller coil just pin point better or what? Can someone educate me? Thanks