I don't own a single firearm so I don't have any experience with this (I mainly mentioned it because I know many of you do).

The site says this about using it on firearms:

Spray on rag and then rub down firearm, leaving a thin film residue which will not leave fingerprints or come off on hands or clothes. Tack-free mechanism lubrication. Use on bolts, slides and magazines. Will not affect bluing. Good for wood-stock.

and from the FAQ section:

Can I use Fluid FilmŪ on firearms?

Yes. Apply Fluid FilmŪ to a clean dry rag, rubbing into metal surface as would be done with a polish or a wax. Will not leave fingerprints or affect bluing. The lanolin base of Fluid FilmŪ will also act as a conditioner on wood stock. Fluid FilmŪ will hold up in extreme conditions.

It is a fantastic penetrative but I can only assume that because you're using it in such small quantities, it wouldn't get into the ammo unless you drenched the gun with it. It's relatively thick and stays in place very well. I use it to lube the chains on my motorcycles and I get next to no fling.