I found a way to get other factions armor researched quite early (if you have the resources). All you need is to recruit 2 soldiers that have the same class from one faction and take their armor off (you need 2 pieces of armor to start the research). That will get the Reverse-engineering research for them unlocked. I haven’t finished it yet, so I don’t know if you lose the armor pieces used for research, better have some spare armor just in case.
I don’t know if it’s intentional or a bug.
“Take their armor off.”
Sleeping your way to some fancy new research? xD
I did reverse engineer other factions weapons and you do lose what you reverse engineer. However you can now make as many as you need and even get the ammo.