Why do people say "metonymy" when they mean "synecdoche?"

Metonymy is when you say "Ceres," but mean "grain." Or even when you say "Hibernia," that noble ivory figure who weeps over potatoes in Punch, and mean "Ireland," that small green island that also weeps over potatoes.

Synecdoche is when you say "keel," and mean "ship." It is not a town in New York.

They are similar, it's true. There may even be some cases in which both apply--"the White House" for "the United States," perhaps, which is essentially metonymy but also literally synecdoche.

But when you say, "Part for whole; you know, metonymy," you sound like a moron. Because you are incorrect. These are specialized terms, with specific meanings. They are rarely necessary to use in order to be understood. One could even say that they can act as an impediment to understanding, especially when used incorrectly.

So don't.