Courtney,
Thanks for coming back. I'd respectfully disagree on calibre - I believe that 20mm is just too small for anything except attacking easy air to air targets or soft ground stuff. Most of the calibres around (including the M61 gun's) are also fairly slow.
27mm is actually a very good compromise and an aerodynamically good round - but when I witnessed Diehl's tests on the HE round I was not impressed with their choice of explosive vs. fragment weight. Bit geeky for some - sorry.
Choice of driving band was driven by breech pressures. The 27mm's very large cartridge generates pressures that (at the time) required the metal driving bands. Lower pressure 20mm cannon were able to use plastic bands earlier. We had the same issue with the Aden 25mm, where still bands had to be used. Tailoring of propellant can do a lot to mitigate the issue.
Sorry to see this thread turn into another 'I hate F-35' thread but there it goes. B and C chose a podded gun to preserve signature when the gun's not required, and to reduce impact on internal fuel and weight. The logic for the B and the C (others won't agree) is that they'll fit the gun as and when it's required and of use. On an aircraft that's got severe challenges on internal space, fuel and weight, it makes some sense to me, but that's just my opinion.
The F-35 customer wanted the ability to deliver a useful weapon load at the lowest practicable (careful choice of words there) signature, as well as the option to carry lots more external stuff when signature was less important. That's what the external pylons are about. Yes, stealth is compromised on that fit. And yes, i know that others argue over what a 'useful' load is. Again, the concept makes sense to me, probably not to others though.
Hope this helps a bit
Best Regards as ever to all those making the choices for real,
Engines