Pictures from Battlesim.com's latest milsim vietnam themed event, pictures taken by myself.
LINK
http://GwagDesigns.smugmug.com/Military%20Simulation
http://GwagDesigns.smugmug.com/Military%20Simulation
Indeed you are correct, it uses custom shortened M14 mags, once modded, they barely stick out of the magwell, as the normal mag would greatly take away from the look.