Are there some class 6 open felonies that are not eligible to be designated as a misdemeanor?

Yes. The class 6 felony cannot be one that involved the intentional or knowing infliction of serious physical injury and it cannot be an offense that involved the discharge, use or threatening exhibition of a deadly weapon or dangerous instrument. Arizona law defines “deadly weapon” anything designed for lethal use, including a firearm and a “dangerous instrument” means anything that under the circumstances in which it is used is readily capable of causing death or serious physical injury.