The first law to pay attention to is ARPA (Archeological Resources Preservation Act).
Although ARPA does not specifically mention "metal detecting", you need to understand what ARPA says about digging artifacts. Understand that the ARPA law applies to State or Federal lands. It does not cover private property.
They Had No Idea They Were Breaking The Law
I recently heard of a situation where three individuals who were digging for old bottles on Missouri State Land and were arrested and thrown in jail. Fortunately, one of the individuals had a brother who was an attorney. After spending a few hours in jail, they were released on bail.
Whatever you are digging, if it is more than one hundred years old, you are breaking the ARPA law. The States don't care whether you are digging for bottles, coins, or picking up Indian arrowheads, to them your breaking the law.
In addition to the above mentioned ARPA law, if you are treasure hunting in Missouri, you must be aware of another law, the National Historic Preservation Act. (NHPA)
A Word About Archaeologists and My TV Series
I produced, and was the host of a metal detecting, TV series, Exploring Historys Treasures.
I was the first to produce a reality TV show of this kind. When we first aired, State Archaeologists tried to shut down our production. They hired attorneys and threatened us with lawsuits. Their argument was that we were digging on State lands. Unfortunately for them we had filmed all episodes on private property with written permission from the property owners. The States and the Archaeologists lied about what we were doing. The fact was, they did not like the idea of our series showing the viewers how to locate and dig artifacts. So be careful. Make sure you do not metal detect on State Lands.
My metal detecting laws page has more information regarding the laws.
If you want to detect on private property, then be sure you get written permission from the landowner.
For detecting in Missouri, town, village and city parks, you'll need to check with those local officials.
I cannot be responsible for any outdated laws from the time of this posting.
Have you tried metal detecting in Missouri ghost towns? My home page is all about metal detecting in eastern ghost towns.