I would do a wash with a dark, earth brown color as a start. After that, it is up to you. You may want to do a bit of dry brushing, maybe some paint chipping, a light overspray, use some pastels, etc. I typically don't like to weather my vehicles too heavily. I think that a good wash, a tiny bit of dry brushing, a light overspray, and finally some artists' pastel chalks for rust on the exhaust and soot around the opening of the exhaust and the gun barrel produce pretty good results.