This video demonstrates the depth capabilities of the Garrett Ace 250 metal detector with the stock coil in mineralized soil. The item being tested is a clad dime due to the metallurgical consistencies in the manufacturing process and because of their smaller size. It is reasonable to assume that the Ace 250 will achieve greater depths on larger coins.
The test box used in this video was designed to provide 3 measurements in each section of pipe with the matching 1″ thick piece of wood. A dime is mounted to the outer edge of one side and in the middle of the piece of wood on the other side. This allows for 1/2″ measurements. Keep in mind that actual depth in the field will depend on local ground mineralization and some detectorists argue that coins buried in the ground for long periods of time develop a halo effect that makes them more detectable. Actual performance in your location may be better or worse than in my local soils.