I've been visiting the National Archives in Washinton DC on and off for over 25 years, but I just stumbled across this article which is full of tips for anyone doing research on military uniforms, equipment, weapons, etc. I had the privilege of knowing the author, Mike Musick, and benefitting from his extraordinary knowledge.
I don't know why a copy of this article isn't handed to everyone entering the Archives who is embarking on research in military history, and I think I'll ask that question next time I'm there.
The article suggests checking many places the average person would never think to look, probably due to the varying quality of the cataloging that has gone on over the years.
To Join the Company of Military Historians click here