I liked the reloading mechanism, could be smoothed a lot though, they also tumbled alot when they left the barrel, often colliding against the tower sideways. Maybe reduce the speed of the shells, that might make it more stable and more likely to collide with the target.
No mechanisms (aside from the simple piston) brings the score down quite alot. Since it's pretty much a thyme operated circle spawner I'm going to have to give it a 2/10.
Aesthetics are all very well and good, but Phun/Algodoo is not an art game.
Forgetting the lack of mechanisms for the moment, I got a couple of tips for the artwork.
A lot of the time less = more. The gun looks too blocky. Steampunk is meant to involve very refined, yet primitive technology.
-It's very thick, slimming it down would make it look a lot better.
-Use more curves, currently it looks quite blocky.
-The jar of chemicals protrudes too much and ruins the shape, maybe incorperate it into the body?
-Try make the handle grip slop backwards a bit, for the same reason as above.
-I see you made an attempt at a mechanism near the end of the barrel. However, it looks too random. You need timed, ordered movements instead of the unpredictable gaps appearing.
-Adding small, pointless mechanisms (even if just a partially exposed rotating cogwheel) would make it look more functional and help fit with the steampunk theme.
All of that being said, I think the shading was done quite well, but the 3d effect was superfluous.