Google does not let you run AdWords to promote weapons or firearms. The guideline text says;
Advertising is not permitted for the promotion of certain weapons, such as firearms, firearm components, ammunition, balisongs (switchblades), butterfly knives and brass knuckles.Now, I think it's fair enough that Google's natural search results return the most relevant websites when I search for [ammunition]. As expected; Wikipedia comes first with an article that educates me about ammunition.
Let's look at the (highlighted) Universal Search related suggestion though. It encourages me to search for [ammunition for sale].
This isn't just interesting because Google doesn't want to help people find ammunition for sale this is interesting because the related search is pushing be into a commercial query rather than a navigation or information query.