With the Cibola, I found running too hot super tuned took away from being able to discriminate out larger iron and from getting some of the subtle characteristics of the beep.
So, with the threshold set so that it can be used to pin point and size targets, I find the Cibola fairly good at discriminating targets and telling me what conductive range they are in.
Like Squaby says, the discriminator knob can be slowly thumbed as you swing over a target. Increase the discrimination until the target goes away, then back it down until the target beep kind of breaks up or is present again. Dig the target and see what it is. After a few similar targets are dug you start to see where the targets are responding to the discriminator in the ground you are searching. You will learn what target types discriminate out where.
Bottle caps can be tough to ID. In many cases they sound pretty good and act like dimes but may pinpoint a little bigger. They do have a subtle clip to the beep when the discriminator is set to foil or above, but I've found the clip on the beep to be much more pronounced on the Tejon than on the Cibola.
The vco pinpoint sound gives indications as to depth and to a lesser extent target type. Aluminum kind of screams and sounds shallower than other targets of similar size. After digging several tabs, you can get an idea of what they sound like pinpointing and where they discriminate out at.
The kicker is that small gold and silver chains act like pieces of small iron wire, nice gold rings and good size chains can act just like small foil balls. Big rings can range from foil up to square tabs. Some large class rings may even sound like zinc pennies.
Indian head pennies act like zinc pennies too.
Even with good target ID you have to develop and understanding of what you may miss if you don't dig and decide for yourself what your hunting priorities are.
You still using the Ace 250?
Cheers,
tvr