Sniper rifles, rocket launchers, and other high-power low-ammo weapons. The launchers might take too much TU to fire, reload, and fire again in one turn, though in some cases a soldier may have enough to fire and reload (without firing again) in a reaction period. Now snipers, especially reacting at long-range from a vantage point, are more likely to exhaust their ammunition in reaction and have enough TU remaining to reload in a reaction period, which would be much appreciated.