Hello, Robert ---
Very nice. It was a good day metal detecting. Your coin is not particularly valuable, but it sure is interesting.
It is from Mexico (the oM mint mark) during the time of Spanish colonization. Charles III (Carolus III) was king of Spain (Hispan). The coin is made of silver. The denomination is one-half real, equivalent to a 5 cent coin in the US. America's denominations followed the Spanish colonial ones:
1/2 real - 5 cents
1 real - 10 cents
2 reales - 25 cents (you've heard the term "2 bits" for a quarter?)
4 reales - 50 cents
8 reales - 1 dollar
The FF letters are the initials of the assayer at the Spanish mint.
The coin in my picture sold to a collector for $68 US dollars during a recent auction. My pictured coin is in better condition than yours, because yours has more wear and has also been damaged with a significant dent. The poor condition of your coin renders its value down to about $10 or so.
For a run-down on coins like yours, from 1/2 real to 8 reales, see this page: