"but I do believe that these two pictures are photos of the 20mm "Vulcan" gatling gun"
You are correct, this is the General Electric M61/A1 Vulcan cannon. The top photo also shows a 510 round ammo drum, although the ammo guide chute is not a configuration I'm familiar with.
The top one (of the chute) looks like it MIGHT have come from an F-15. I may be wrong, but that's about the fig' they had (and it's been 20 years for me).