grammatical number - Plural followed by singular - "have" or "has"?

I'm wondering which of the following is correct:

Guns are an invention that have had an enormous impact on African history.

or

Guns are an invention that has had an enormous impact on African history.

These sentences aren't actually being used in my report, but I decided to create them to save you from the grammatical horror of my actual run-on sentences.

Anyway, I know "have" is used for plural subjects and "has" is used for singular subjects (wrong term, I'm sure), but I'm unsure as to whether the subject is the plural "guns" or the singular "invention." Microsoft Word is suggesting the latter sentence. Is that correct?

Answer

The subject of the verb is an invention - which is singular, so the verb-form should be too.

Here are a few written instances of "are an invention that has". Note that Google Books contains no examples of "are an invention that have".

Don't bother even thinking about what Microsoft Word recommends or queries - at best it might be useful for flagging up glaring typos/etc. Beyond that, it has no credibility.