In reply to:

http://humanities.byu.edu/rhetoric/silva.htm

Thank you for the link, belligerentyouth. I think I found an error in the section on synecdoche.

The rustler bragged he'd absconded with five hundred head of longhorns.
Both "head" and "longhorns" are parts of cattle that represent them as wholes

Am I mistaken in thinking that, since "longhorn" is a specific breed of cattle, the rustler may not have been using two instances of synecdoche? I read the statement as defining the type of cattle he stole, "longhorns", as opposed to Charolais, Angus, Hereford or Guernsey.