That will be dependent on the exact make, model, and condition of it. We'd need this information in order to be able to answer this question.
The closest would be 18 gauge which is 1.27 mm
18 gauge solid copper wire is 40.3 mils in diameter, that is 0.0403" stranded would be larger.
18 gauge
18 gauge is thinner than 10 gauge.
18 gauge is thicker than 20 gauge. 18 gauge is 1.27 mm thick while 20 gauge is .953 mm thick.
If available, it would be a custom build.
14 gauge is 2 sizes bigger than what you would get your ears pierced with (18 gauge). It is the standard gauge for most piercings.
16 gauge is thicker than 18 gauge.
An 18 gauge is not impossible. Between about 1860 and 1915 almost every size was used at least once by someone-somewhere, and someone else-somewhere else would probably pay a premium for an unusual gauge. However, an 18 marking would more likely indicate the choke size. I believe that would be a very tight choke on a 12 gauge. Are you sure there are no other markings? Is it a single shot, double barrel, pump action, or what? If it is a single or double, does it have have hammers, and if so are they on the sides or back of the action and is the main part of the action behind, above, or in front of the triggers? Your question is about like "What is a car with a 3 cylinder engine worth - if there is such a thing. I can't even guess at the value without knowing what it is.
18 Gallons
Around $100 to $150 used good condition
50-200 USD or so