I have a Donnington 15mm artillery model, I think it may be OG01 Ottoman bombard. Most of the length of the barrel is corrugated as if it was wrapped with cord or wire. I am not sure how to paint this, because I don't know what it represents. Is it some form of reinforcement wrapping? If so, would it be rope, copper wire, or something else? Or is it just meant to be decorative grooving on the barrel? Any ideas?
Those look to me like reinforcing metal rings. The key feature is that each ring appears to be 'sealed' on top, which suggests they are made of a non-malleable metal, most probably iron. This in turn would indicate they are functional, not decorative.
Reinforcement of this nature would help to prevent a 'flower pattern' burst at the mouth or a longitudinal burst nearer the breech.
I suspect Patrick is right, though I've not seen a picture of a bombard made like this before. Normally, the reinforcing is flat iron bars and they abut each other, rather than the looser round bars here. That said, I've not seen much smaller Ottoman artillery - its usually the big cast bronze ones that feature.