Oh I know the answer to this!
In our state (MA) they use the smith and Wesson MP15. It’s a .223 cartridge. They use them because at one time S&W was a state industry; most former military could pick it up and use it effectively with little training; and they are inexpensive.
I don’t know about other communities...but that is standard in most MA Police Departments.
Is the weapon marked "5.56mm" or ".223 Rem"?