F-35 Gun
LO,
I was on the project when the gun decision was announced. At the time, I thought that the decision was largely driven by domestic concerns. The life cycle costs arguments were not all that convincing, and the effectiveness arguments even less so. The LM analysis even ignored the volumetric hit of a Gatling, which given the fact that the gun displaced fuel tank space was surprising.
The angle of the gun was driven by the need to get the shot line clear of the upper fuselage after it left the muzzle. The hump was driven mainly by the need to cover the multiple Gatling barrels and the bulky aft breech housing. There was a multipurpose 25mm round available, I happened to know because I had bought it for the Aden 25mm on the GR5 four years earlier. When I mentioned it, the LM armament engineers said they'd never heard of it. The same round was also available for the 27mm cannon as well.
In the end, I believe that the clincher was the fact that H&K had not, at that time, been able to demonstrate a working prototype for the 27mm's linkless feed system. Given that the LM team were frantically trying to reduce risk, that made it an understandable decision.
Hope this helps
Best Regards as ever
Engines