Hi Tony,
>As I've said before, the major determinant of dispersion in RL would have been aircraft movement in flight, followed by the rigidity of the gun mountings, followed by the accuracy of the gun. The first two are so significant that I don't believe that anyone would notice any differences in the third.
Actually it was possible to point the aircraft nose with very high precision. Lopez in "Fighter Pilot's Heaven" describes camera gun runs with a P-38 which shows that he was tracking the practice target with about 1 mil accuracy in the best recorded runs.
The Luftwaffe measured the accuracy of their guns, and they arrived at very different values for the different cannon, with the MG FF/M being twice as accurate as the MK103 when they were both mounted in the nose of an aircraft.
Of course, the position had an influence, too. Due to an ambiguity in the German records, I'm currently not sure if it increased from 1.9 mil to 2.5 mil for the MG151/20 or whether I have to read this as 0.95 mil to 2.5 mil, but it certainly made a difference.
(When you're mentioning dispersions, are these dispersion radii or diameters?)
>I do have hard data on the accuracy of the .303 Browning as installed in the wings of the Hurricane and Spitfire; these weapons achieved 10 mils dispersion, with 75% of the shots within 5 mils.
>Which prompts the question - does it matter, since it's the installed accuracy which counts?
For us, it matters, since the same Brownings mounted in the nose of the Mosquito will fire more accurately :-)
>P.S. I've just checked the gun pattern diagram for the Spitfire V included in 'Flying Guns: WW2'. This indicates the shot pattern for the guns at different ranges. I don't know how accurately they drew the dispersion circles, but the .303's scales to 0.5m at 100m, the 20mm to 0.45m.
Thanks! If these circles mean anything (for the Luftwaffe diagrams, they were not equal to dispersion radius, which was given as figure in the operating procedure), that would confirm that the Hispano II was slightly less accurate than the MG151/20 since the latter was expected to land all hits within 70 cm x 70 cm at that distance - slightly less than the Hispano's 90 cm circle. (Note that despite a specified dispersion of 2.5 mil, for passing the gun calibration test 3.5 mil were allowed.)
However, I'm not sure that this is a valid interpretation since the 7.7 mm guns with a 10 mil dispersion would have required a greater circle then.
Regards,
Henning (HoHun)