In the place M.M.L.(My favorite)There are 3 buildings with snipers. One has windows, and 2 have open tops. The open top with a shorter ladder has a 1 hit kill sniper on it. Bring a shotgun with you and then snipe people, and when someone comes up, shoot them with the shotgun. Ammo appears for the sniper about every 40 seconds or less. When someone comes up kill them and salvage the ammo (if rebel).
P.S. At the bottom there's a Mantel shotgun.