Davidson Seat Depth Checker
I use this one for $14.
If you find inconsistency in measurement, Take a bullet that you are most likely going to be using, coat it with JB and lightly spin it in the tip of the checker. Measurements are very repeatable, I do base/ogive readings on the bullets. You can do bearing surface if you get two, but I've quit doing that. Tool is also invaluable to document loaded round OAL (to ogive). I don't use the base attachment for the cartridge head, just the nose piece.
Bullets have really come around in the last two years. Three years ago I was finding .020 variance this year it is down to somewhere around 90% +/- .001 8% +/- .002 2% +/- .003
Thanks goes out to Hornady, Sierra, and Lapua!