Testing done by the USAAF found that the bullet pattern from a B-17 during ground testing had the following results for 12 rounds to 600yds:
ball turret > dia. 15' - 8.3mils
upper turret > dia. 21' - 11.7mils
chin turret > dia. 23' - 12.6 mils
waist(closed) dia. 26' - 14.3mils
side nose > dia. 34' - 18.7mils
tail turret > dia 45' - 25mils
For the B-24 it was:
ball turret > dia. 15' - 8.3mils
upper turret > dia. 20' - 11.2mils
nose turret > dia. 23' - 12.9mils (Emerson)
nose turret > dia. 35' - 19.3mils (Motor Prod.)
waist(closed) dia. 23' - 12.9mils
waist(open) dia. 63' - 35.6mils
tail turret > dia 35' - 19.3mils
taken from: "Gunner" ISBN 1-55046-332-2
scroll down,
http://history.amedd.army.mil/booksdocs/wwii/woundblstcs/chapter9.htmtail gunner - 13.6%
ball turret - 10.6%
top turret - 10.6%