A coil made for detecting metal will detect metal. All metal.
I used an MXT yesterday in the fresh spring air on a 120 year old homestead (Trashlandia)with a HotFoot coil. Many firsts, including a nice brass padlock -very old. I used the small coil because of the brush, scrub trees and forest. The same type of environment would be found in gold country, but not all. The value of a small coil is important to any operator who would encounter such places. I am impressed with the reports of the XTerra but still would want a small coil no matter what the reasons one is not available now. To imply that this machine and a system of coils including the one in the illustration is expected,... requires no defence for the maker! I would go further to say that it is false advertising. I inquired about coil availability and found that the after market production of these coils would not happen. My watch has more smarts in it than the chip sets used in this system and I do not think the after market coil makers would want to entertain making these chips, if indeed Minelab ordered these from some burger-king chip plant. So that statement blows the proprietary excuse out of the water (too costly is what's proprietary). The nice thing about the chip-sets is that you could run a long extension from the head, eg. trolling for meteorites or gold in the desert. Did they use an 'off the shelf' chip-set or had one designed? Does not matter to me. You cannot null out the imprint from the chip in a small package, if you do you will lose baseline performance of the coil. Good engineering knows its limits but marketing does not. Now the bee is on the engineers and I will gladly encourage their efforts as they have to defy the science they are disciplined in. I like the machine but yesterday's finds would not be possible with the existing coils and configurations available now. Well,... maybe what was next to the lock from a distance.
I can understand not wanting a smaller coil if it does not suit your needs. I never understood the importance until the need arose and then I learned how important a small coil was. Even if the present stock coils are good on small gold, a detectorist going to look for gold would have a small coil for the places that warrant it -regardless of the type of machine used(pulse/VLF). Ever use a pin-pointer? Most gold discoveries are small and in larger population than the big lunkers. Use the right tool for the job at hand. That's why many gold machines went to a higher VLF frequency- they 'resolved' the small stuff! 18.75 kHz is a balance.
Coil size: A smaller coil would 'see' the smaller targets while the larger coil may 'look' beyond in a greater, less concentrated electromagnetic field. One responds to the smaller proximal targets and another has greater depth of detection for the larger. You gotta have a ping! It is a balance. Sometimes the ping is a big 'catch-ing'.
Do the BB tinfoil test at 3kHz. Best test to learn that coil's ways.
Happy hunting,
Cannonball