here's a different way to handle the motorcade: first, i hate waiting all day to get a militia-man out of his smelly ensemble; so i dash to the contact point, grab the M195, and drop it just outside the alley where a passing soldier will "find" it. it makes a great lure. it's tough to sneak up on your prey though, as his head goes on a swivel like a tracking radar as soon as he's standing over the weapon, so i'll typically opt for a 9mm SD headshot. you can also use the silenced-ballers, which are also handy for throwing the carcase deeper into the alley so you won't have to drag it as far to hide it. bad thing about ballers is the multiple-shot aspect which can work against your silent assassin rating.
switch clothes, pick-up the M195, and climb the ladder that triggers the motorcade. get off the high ground and head south to the alcove on the other side of the street with the stacks of boxes. lurk behind the boxes until the motorcade enters the city, then you can slide sideways and take aim down the street. i know, seems like you're right out in the open, but the lead UN jeep has never alerted on 47 there as it drove by. i like to crouch and then put the vertical crosshair on the edge of the last building and the horizontal crosshair on the window ledge. it's a very quick shot, especially on the professional level; so i prefer to use the second magnification level: the third is too tight to time the shot. the target's head will be in sight almost immediately after the driver's.
if you miss the target but hit the windshield, the driver stops the vehicle and runs and you get a second chance, but you really need to hit the target. seems like the UN soldiers always kill someone extra if you take the second shot. triangle out of magnification, drop the weapon and head south, avoiding the soldiers as much as possible. it's very hard to get SA on the prof. level, especially since you can't save and take the shot over and over. in this case you may be better off taking the shot from atop the trigger point.
if you've already played through the level and can start with the M195 you can forget the contact point and sneak straight to the alcove without a disguise, drop your weapon, climb the trigger point, and continue. a fairly quick way to get SA after adding the rifle to your collection.