Complex sentence: The milliradian (0.001 rad, or 1 mrad) is used in gunnery and targeting, because it corresponds to an error of 1 m at a range of 1000 m (at such small angles, the curvature is negligible).
Split into simpler sentences: The milliradian (0.001 rad, or 1 mrad) is used in gunnery and targeting. It corresponds to an error of 1 m at a range of 1000 m (at such small angles, the curvature is negligible).