Based on BHD's portrayals of Delta SSG Daniel Busch (I think - left, Colt 727) and SFC Randy Shughart (right, M14 - cf. post #1529093 ).
Edit: to clarify: I'm mostly basing the ID of Busch on the carbine's paintjob, as Busch's in the movie seems to be the only one with a painted front sight block.
Edit: Also, is there a tag for the MD 500/520/Little Bird/etc. family? Closest I can find is hughes_500d, which only has one post, and in any case refers to only one specific model in the family.