> I wonder why they didn't just use 20mm.

Canada tried that once, with poor results.

https://news.yahoo.com/weather-balloon-went-rogue-almost-161...

F22 service ceiling 50k ft, M61A2 vulcan 20mm effective range 2000ft, balloon was at 60k ft

Ceiling for F-22 is 'above 50k' feet. I think the balloon was at 60k.

Those 20mm will land… somewhere