Something that may help you is to make a Test Garden.
Bury at 2-4 inches deep for instance, assorted coins, a few different makes and shapes of pull tabs, bottle caps, pop cans, tin cans, a nail, bit of steel and copper wire, brass spent shell casings (.22, .44 mag, 30-06, etc.), lead and copper jacketed bullets, lead fishing sinkers of different shapes, you get the idea.
It is my understanding, lead at about the same size as a piece of raw gold, will detect about the same. If for instance you had a lead .22 caliber bullet and can detect it, if there were a piece of raw Gold at about it's size, you should also detect it. Different machines, coil combinations, the frequencies they operate, the ground minerals they are used over, are limited to just how small of gold they will detect. The operators techniques are also to be considered. Gold detecting with real Gold Machines is not the same as coin and treasure hunting. Is a whole different critter. I notice many will make a Post heading of "Found Gold" but when you click to read their Post, it was not a nugget but a ring.
Make a map of the items you bury and keep them spaced apart enough your coil(s) are not detecting two or more items at once, and make sure where you make your garden, no other metalics are present. You can even bury coins on end to make it more challenging. You will also find the longer they are buried and allowed to tarnish, oxidize and or rust, the more your Test Garden will best represent the conditions you will hunt except that the mineralization changes from place to place or even in a matter of a few inches and feet. If air testing, the machine has no ground minerals to adjust to so will not be representative of your actual hunt.
My Pioneer 505 is the only machine I have with tones (I'm old school detect by sounds) so I'm going to warn you, do not have total reliance on the bells and whistles and do not be discouraged with them when not correct. These machines are set up for what coins and such, and the soil conditions you will most likely encounter, and the readings are a tool, and can be fooled, as any other brand of machines, by outside of the norm metal alloy contents and shapes. I have been detecting since 1982 and it never fails, you will run into something that will fool you, by sound only or the machines with bells and whistles. Small coils will help a bunch in trashed out areas but you must compensate by greatly condensing your swing to get proper coverage. You will also find you can get closer to metal posts and such than with a larger coil. Depth will also be reduced but you will also detect smaller items. One old rule of thumb for depth of round coils is the depth is equal to the diameter of the coil. Your coverage is in the shape of an upside down cone. You can imagine how tight now your overlap must be to get the most coverage related to depth as possible. Some say that end point is really about an inch on the larger coils. With a round 7" Garrett coil though, I once found a 5/8" dia. survey stake at 14 inches. I would not have believed it if it were not me that detected and dug it while a friend watched and then measured it. For a person who had never detected before, he bought himself one just because of that.
Practice, practice, dig and practice. Many times anymore, I use less discrimination than I used to. You will get the hang of it and read all you can find on the kind of detecting you seek to do.
