Instructor Course
I would highly recommend this course. Once you've taken the NRA instructor's course, you'll have a level of "expert documentation" which goes above the average joe - despite the poor quality control of the NRA instructor training program (flame away!!).
That is not to say that the specific course referred to here is poor quality - just that the NRA oversight of what it takes to become an "instructor" is weak at best. All of that being said, NRA Instructor is the one non-law enforcement credential most likely to be accepted as evidence of your competence..
You even get to sign RI carry permit applications if you can find some Army-L targets

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Consider some other disciplines :
Scuba - You don't become a recognized instructor by PADI, NAUI or the YMCA without an extensive amount of training, experience, and verified expertise.
Sky Diving - Ditto.
General Aviation - Another ditto.
Handgun - Basic competence; 2 day course; no experience teaching required to get the certification. You can go from newbie to "certified NRA instructor" in less time that it takes to get a card to have your scuba tanks filled; sky dive without a jumpmaster; or solo a small aircraft.
Interstingly, the NRA has started to apply what appear to be "real standards" to their "refuse to be a victim" program, including doing actual teaching under supervision or providing a tape of your presentation skills to NRA HQ.
Rob