I am strictly a Civil War relic hunter. In the 3 months since I've had my MXT300 I've found dozens of Minie balls (average depth 9-10"), a dozen or so iron artillery frags (some 15"deep), and a complete live 10 pounder Union Parrot shell. I also found 1 Confederate Virginia state seal button and a North Carolina sunburst button and numerous other relics. These came from a completely "hunted out" battlefield in West Virginia.
I used the 12" coil that comes with the 300. I set the gain at +2 or +3, the threshold at about 1:00, and the disc at 3. The deeper bullets will not ID properly on the meter therefore you have to go mostly by sound. Using the crisscross pinpoint method many of the bullets would read small iron (low tone) on three of the sweeps and the maybe on the fourth sweep a higher tone say from 12 to 85 on the VDI. When you get one of these funky signals check the depth and if it reads 7" or more dig it.
That being said I just bought a 10DD coil and only had a few minutes to use it at this battlefield site last Friday. I went to a spot near a small cattle barn which is now in the middle of where some of the heaviest fighting took place. This place is loaded with semi-modern nails so I thought it would be a good place to try my new coil. Within 5 minutes I found a fired 3-ring Minie ball in an area where I hunted with my 12" coil the week before and my buddy using his Fisher's CZ5 also hunted the week before. The 10DD spotted it just fine in that mess of old rusty nails.
I can't wait to get back out there later this week when the rain stops. I hope this helped.
Greg