I don't get the old school thing, but:
I think the barrel could look smoother, i know its supposed to look old, and beat up, but the barrel's dithering is a bit too much. The yellow skull symbol shouldn't be outlined imo. I also think having less contrast between the brightest shade you have now and the rest of the piece would be a good idea. Maybe you could use that shade for rust?
Also, where is the lightsource on this exactly?
Over all its not bad.