Some form of non-burning acid, maybe like salicylic acid or something a little stronger would be great to put into a pellet or paint gun type of shooting device. I’m not sure how well this would actually stop someone or how much it may just make them madder; since it would hurt more than deter the person from still attacking you.
Maybe someone knows a good combination of topical chemicals which would be good enough to pass as safe for FDA and ATF. But I’m not sure if the Firearms and Tobacco (whatever they are called now days) Agency would even care if it was dangerous or not. Maybe that is a point of conversation that could be discussed as well: How does the ATF “regulate” what is shot from a firearm? Or do they?