Hey Robin ,
that script should work I opened a new game to work it out and the only thing I can come up with is that maybe your vocabs are set different.
shoot is Imperative Verb
gun is Noun
bill is Noun
I've tried several different phrases and they all seem to come back correct, "shoot gun at bill", "shoot gun to bill"
"shoot gun at wall" all of them worked, so like I said check your vocabs, sometimes when I add a new word I forget to add the type, make sure gun isn't set to something besides noun