I think that there should be 2 pages of upgrades, one holding all the "add on" style ones like extra cannons, armor and so on. The second being for specific main cannon upgrades, stuff like flatness (trajectory, not soda), damage, damage over time, cooling, and so on. Also, try adding in some game mechanics to hinder players, that will have corresponding upgrades, like semi-large spread and/or a low fire rate (once high enough give 'em full auto).
I think i'll shutup now.
Nice graphics, and the shop system is a huge leap from the last one. Oh, and your guns should overheat, but before they do that, bullets should torn orange or red, make a molten metal splash effect on impact, and do 150% damage :D.
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