Questions? What coil were you using and did the half dime show up under the pinpoint spot or was it off to the side. Did you dig any trash along with it?. At only 5 inches and the ferrous jumping down to 27 sounds like it was on edge or had trash near it. I dug a silver wedding ring that mirrors your detector reaction on your half dime. The only difference was my ring was down around 9-9.5 inches. Your dime is a great find. And that's exactly why I run my ferrous disc at 27 when hunting an area that has aleady showed great promise. I had dug an 1845 seated dime the day before I dug the silver wedding ring. The only bummer with the ring is it didn't have a single set in it but, oh well.