first, this looks like an awesome project, good luck!
second, 80/20 makes
linear slide bearings (so you don't have to make them yourself)
third, if you paint (or add graphics) to the chambers / barrel that look like soda cans, i think the proportions would work better to have it look like 2 or three cans end-to-end. Basically, grab a can or two and look at it, 'cause i think the proportions would look odd if it were painted like a single tall can for each tube.
I'm excited to see this thing built!