On the motorcade level, you need to clear the town quickly and grab the supplied rifle. Shoot your target. This will cause the UN guards to stop and run on foot to your position. Wait until they get through the gates, then dodge their shots and put them in the cross fire from the remaining troops outside. Since you cannot hurt them, do not shoot. They will soon shoot their own men. Then, just run up and grab an M4. This may not be easy, but it does work. To get the M4 easier, just use the fiber wire on the UN guards.