WARDOGS uses real-world ballistics: bullets drop and take time to arrive. At close range neither matters. Past a few hundred metres, both decide whether you hit.
Ranging with your reticle
If you know how tall something is, its apparent size tells you how far away it is:
range (m) = target height (m) × 1000 ÷ height in mils
A standing soldier is roughly 1.8 m. If they fill 3 mils of your reticle, they're about 600 m away. Crouched, call it 1.0 m; head and shoulders over cover, about 0.5 m.
For distances between two places on the map, the WARDOGS 3D Map by N4 Lab can measure them for you.
Zeroing and holdover
A rifle zeroed at 100 m puts the bullet on the crosshair at exactly 100 m. Further out, the bullet lands below it, so you either hold above the target or change your zero. Most rifles in WARDOGS let you adjust the zeroing range (often 100–1000 m); see the controls for the keybind.
Leading
Travel time is the number people forget. A 900 m/s round takes about two thirds of a second to reach 600 m. Someone sprinting sideways at 4 m/s covers about 2.7 m in that time, so aim well ahead of them, not at them. Each weapon's muzzle velocity is on its weapon page.
A word of caution
The maths above ignores air resistance, so real drop at long range is a bit more than it predicts. Past about 400 m, check your shots on the range.