Seems I'm a bit late on this post, but "expert" is defined as:
1 (obsolete) : EXPERIENCED
2 : having, involving, or displaying special skill or knowledge derived from training or experience
synonym see PROFICIENT
So, to me, for someone to be an expert, he/she has to have a special skill or knowledge about something (in this case, something gun-related), which comes from training and/or experience. They don't have to have skills and knowledge in everything related to guns, though, to be experts. They could specialize in some particular subject (gunsmithing, marksmanship, history of a manufacturer, etc). An expert can be specialized, focused on one thing.
As far as instructor qualifications, I would think that they should be experts in what they're going to teach. And they would have gotten this qualification by training to do it, or from many years of experience. You don't have to know everything about everything to be a teacher. You just have to know your subject matter and be able to convey it to your students so they understand.
I consider the sergeant at my range who showed me handgun rapid fire techniques to be an expert, but only at rapid fire w/ a handgun. He may know nothing about rifles. Unlikely for being in the Army, but it's just an example. For me, I guess you could call me an expert in using AutoCAD (from both training and 15 years experience), but there's still a lot of the program I don't know about.
Personally, I am neither an expert nor an instructor. I'm a willing student, thirsty for knowledge and improved skills. Technically, I'm probably still a novice, but I'm trying to get better. Practice, practice, and more practice.
Dave -=|=- Please visit my Gun Stuff
page (my tiny speck on the 'Net)
1. Springfield Armory XD-9 sub-compact (1110 rnds as of 06/18/05)
2. Springfield Armory XD-9 sub-compact (w/ blue & silver GK, gift for fiancee)
3. Ithaca Gun Company Model 37 12ga shotgun (from Grandfather)
4. Future: AR-15 build