I don't think I've ever saw a Nautilus at the DIVs I've been to. With how the Nautilus audio is setup...I think you could hear the targets if it is set to zero discrimination. I haven't ran a Nautilus in several years but it seems like I don't recall it having a true all metal mode, but rather an audio filter that worked based on where you set discrimination at. Everything you reject will come in thru one audio channel and everything above the reject level will come through the other audio channel with a different tone. In good ground where targets read well, they do well but in the red stuff you would have to set disc to zero and dig everything that beeps. Without a true all metal mode it makes it difficult to size up an object to get an idea of its characteristics. I know on the F75 platform...a minie ball will read just like a square nail with iron VDIs between 11-13. So numbers wise....there's no difference. Audio wise in discriminate mode at zero disc...the audio filter to disc mode will condense the signal to one very small spot and the signal will ID and even sound like a nail. In true all metal mode on the same machine, the target ID numbers will still be the same but without the signal compression audio filter, you can get an idea of the size of the target and by that you can tell a bullet from a nail 90% of the time. The nail giving a much narrower signal vs the bullet's quarter size profile.
Regarding the XTerra series....you would be in a similar situation as above. With all disc notches at zero, you still have the audio filters that wont give you a true sizing ability and you will be forced to be digging everything. What you can do though is utilize the prospecting mode and try to figure that out. Some of the MXT guys do that with their machines and do quite well. The Xterra's prospecting mode and MXT prospecting mode are very similar in behavior. The key is just realizing most in ground targets of mid-high conductivity that normally reads good....in red dirt will read as iron and if you're rejecting iron, you're missing 98% of good targets. I remember my first DIV I dug a breastplate at just 8" deep laying flat in the ground. It is super high conductivity...well into the silver range on id machines...yet it read like a nail all the way ID number wise but the all metal side let me size it up and I still thought it was a bullet. I never would have thought something as big as a plate laying flat and that shallow would still read as iron but that's how it works.