I have uploaded on blendswap a simple cycles shader which you’ll find helpfulI if you are using some softwares like substance painter or quixel suite : just plug the maps and you are done !

This shader works in the classic specular workflow and in the fashioned metalness one.

Here is a comparison from marmoset website between those two workflows :

I have added a roughness view dependant factor after watching this video (at 59:00) :

It can be improved, simplified if you need less features or complexified if you need more, you can do what you want with it as it is under CC0.

Here are some cycles renders made with models found on the net.

Cerberus asset found on eat3d

AKS-74U and Scifi Helmet from Quixel website

If you find an error or an improvement for this shader just let me know in the comments.

Here is the link to the file on Blendswap.