Often, shotgun shells have the maker's name on the head. Unless someone recognizes this particular one, you will probably have to call it a "generic 12 gauge." The shell made a nice signal, didn't it? Like a big coin. I hope you find some of those in your future metal detecting. Welcome to the hobby and TNet!
Originally they were all brass, in 1870's they started making them from paper dipped in wax, then in the 1960's plastic ones were introduced. May give you a clue to age of there is more than the brass head left. If plastic you may still be able to read it.