I don't this works in this way. Maybe the distance between Lara and these guys matters to decide shoot or CQB. Or maybe it depends on if you're shooting in return.
I replayed TR4 last year, and cannot remember guys who are shooting only. I remember when tried shooting in return they grabbed their swords and beated off my bullets.
P.S.: I googled this and found that there're 2 types of these enemies -
http://tombraider.wikia.com/wiki/Desert_Warrior - I beleive they have a bit different behavior.