"Over bore" is a complex term that refers to a firearm's bore diameter larger than the standard specifications. The term has no direct antonym. However, there are a few related words or phrases that are antonyms in specific contexts. For example, "undersized" might be an appropriate antonym in industrial manufacturing, whereas "under bore" could be a potential antonym in gunsmithing. Additionally, words like "standard," "conventional," and "traditional" could also serve as antonyms in some sense. However, the precise antonym of "over bore" depends highly on the context and the intended meaning of the term.