https://forums.eagle.ru/topic/153180-reportedgun-dispersion/page/3/"From "Flying Guns – the Modern Era: Development of Aircraft Guns, Ammunition and Installations since 1945""
The M61 is capable of about 3-4 mils when internally mounted, although the centreline gunpod used in the F 4 is less rigid and can manage only 8-10 mils. (one mil equals one metre dispersion at 1,000 m)
Here is the whole passage.
"Accuracy of the guns varies depending on the weapon and the installation. It is measured in mils (one mil equals one metre dispersion at 1,000 m). The .50 inch M3 could manage about 5 mils. The four Mk.12 cannon in the F8U were regarded as inaccurate, reportedly achieving only 12 mils (or 3.6 m at 300 m). The F 100 with four M39 cannon could get all of the shots within 8 mils and 75% within 4 mils. The M61 is capable of about 3-4 mils when internally mounted, although the centreline gunpod used in the F 4 is less rigid and can manage only 8-10 mils."