Good catch. Even with your excellent suggestion....it makes me wonder if the word "necessary" might ultimately be challenged. I am sure there is a DA somewhere who wouldn't consider my back-up gun "necessary"....(or some other such nonsense).We can replace the words "incident to" with "necessary for" and you get a clearer picture.
Perhaps better to just leave at it "For".
(3) the lawful carrying of firearms or other dangerous weapons in a Federal facility FOR hunting or other lawful purposes