Years ago, I made video discussing a news headline involving a U.S military personnel who assisted the production of Metal Gear Solid for the PS1
MGS1 was the only game in the series to receive U.S. Army assistance, no other Metal Gear game (as far as their end credits suggest) had apart from the series' longtime military advisor, Motosada Mori. Motosada Mori served with the JSDF as well as the French Foreign Legion
Throughout the years, he wrote several books about various aspects of military training and gear.
...and one strange book depicting torture on it's back cover
He featured in many DVDs as well
More information on Motosada Mori can be found here
Motosada Mori was also credited for writing the script to the Metal Gear Solid Radio Drama from 1998.