Postwar analysis placed the overall accuracy of daylight precision attacks with the Norden at about the same level as radar bombing efforts. The 8th Air Force put 31.8% of its bombs within 300 metres (1,000 ft) from an average altitude of 6,400 metres (21,000 ft), the 15th Air Force averaged 30.78% from 6,200 metres (20,500 ft), and the 20th Air Force against Japan averaged 31% from 5,000 metres (16,500 ft).[46]
Many factors have been put forth to explain the Norden's poor real-world performance. ...
from https://en.wikipedia.org/wiki/Norden_bombsight#Postwar_analy...