Have been able to go out 3 hunts since this original post and have done some testing. The beach I was previously going to and spent a solid 2 weeks at had a very high concentration of bottle caps. I have visited 3 beaches since then and there are far fewer bottle caps on average. I have learned how to distinguish bottle caps nearly all the time using the following methods:
Dry Sand or Land:
1. Run Park 2 and multi frequency for best gold capabilities.
2. Set discrimination on whatever you want. Bottle caps can read from 4-18 so it doesn't matter what your descrim is.
3. Once you get park 2 set up how you want it set the custom user setting (button on right side of machine) to the exact same thing except set the frequency to 40k. This way you can switch from multi freq to 40k quickly.
4. When you get a good signal that may be a bottle cap switch it over to custom user (which is essentially just bumping it to 40k frequency) and see if the number jumps around. The number won't jump on multi but will increase in 40k if it's a bottle cap. If it isn't a bottle cap (non ferrous) then it will stay the same number when you switch to 40k.
Still need to investigate this more but those are my initial observations. Was able to get 3 golds in the last 3 hunts at a beach that I was told was not very good. I guess it's all in the machine =)