Generally, bear in mind that the details will only be about 1-2 pixels big on screen, so the extremely minimal version could have the "tubes" of the barrel as 5, 4 or even 3 sided. There's no need to make them hollow. Anything that's circular can be simplified to an octagon or less, the detail on the handles will not be visible, so may as well be rectangles etc. Don't round of the corners of any of the sections. Any minor details can be painted onto the texture (or normal map, should we manage to get them working).
That's probably down to the extremely low level, but there's likely a compromise halfway between that and what you've already got which will be an acceptable poly count without looking too bad.