Meowhead,
Actually, the model 18 has some differences that are designed to prevent one from slapping a model 18 slide on a 17. The 18 also has some other dimensional differences. They use the same mag and most fire-control components, but many other factors differ.
My sources do not indicate a factory full-auto other than the model 18 being available at this time. As indicated, aftermarket conversions to exist, however.