I'm an Adapted Physical Education specialist for a large school district here in Texas.
I work with students who have gross motor delays or require accomodations or modifications to the general ed P.E. curriculum.........basically a Special Ed PE teacher.
(I'm also an FFL)
Not only is what you propose legal, but it is commendable.
Just a couple of observations:
"Handicapped" is NOT the appropriate term. Just as "retard" or "cripple" are hurtful, the term "handicapped" is not accurate either. Special needs means special needs.....and many persons while developmentally delayed are certainly not handicapped. The preferred terms are "people first" as in "a shooting buddy with a cognitve disability" rather than "a retarded guy I'm going to take to the range". It might seem to be based in political correctness, but the term but is actually a more accurate means of identifying the persons abilities. If it were your child would you refer to him as mentally retarded or as a child with a cognitive delay?
Similiarly, saying "he's confined to a wheelchair" for a wheelchair user is limiting language.....I have students who USE a wheelchair and can easily get out and kick your ass if your refer to them as handicapped or confined.
As a previous post mentioned, the nature of the disability will affect what you can do at the range. They may exhibit cognitive, social, behavorial or motor abilities at a stage of development far below their chronological age: example: a thirty year old with the cognitive abilities of a typical second grader. If so, practice your firearm instruction just as you would with any second grader.
If the person has been diagnosed with an autism spectrum disorder, they may have difficulty with the noise of shooting. I have several students who wear hearing protection while in the gymnasium- too much noise or noise of certain frequency is over stimulating, distractful or even downright intolerable.
I have several students who read & practice "social stories" before doing an activity. Example: Before going to the range. WRITE a short step by step story (with pictures if possible) of what you are going to do, how you will do it, what it will sound and smell like, how the person should behave, what are the safety rules and the consequence of misbehavior.
Ideally, you would provide the story a day or so before the range trip and again immediately before the trip.