In the FAQ it says affects enemies weapon range.
For example like the 2 replies before mine:
Melta gun with 12 inch range gets cut to 6 inches. The melta special rule gets cut down to 3 inches, I believe 6 inches is the lowest range the night shields give the effect.
EDIT: Page 63 states, Rapid fire range, half range for a melta shot, and so on and they have no effect on template or barrage weapons