Azure,
The 18.2 on both barrels represents the bore diameter, expressed in mm, 9" from the standing breech. It clearly identifies the gun as a 12 gauge, will tell you positively if the bores have been honed in the past (I doubt it, from what you have told us so far) and will tell you how much choke either barrel has if you measure the constriction at the muzzle with a dial indicator.
18.2 is about .717 in inches. Standard 12 gauge bore is about .729. The gun comes from an era when wads were typically felt or horse hair, and while it would still work well with those, you will typically find plastic wads in ammunition today.
I have lengthened chambers on older sliding breech guns, and advised others to do the same. The barrels on this type of gun are usually constructed much heavier than barrels on other types of shotguns (minimum wall on my little R10 12 gauge is about .060, and goes up to .090, very stout for a double gun) as there was less metal elsewhere in the design of a sliding breech gun, and because French proof is, by law, the highest in Europe. But, I'm more sympathetic these days to leaving original guns in original condition, and altering the ammunition to produce desired results.
You will be well served going either route, and you can make the choice to suit your needs.
As an aside, you would simply not believe how many times in the last 30 years I have heard a similar story about a Darne coming to the US in the same fashion that your example did. I wish I had a dime for every single time I have heard that, or seen it written.
Good luck. I hope you can put the gun to it's intended use from time to time.
Best,
Ted
