Karl,
I vaguely remember sticking a dial caliper into both bores when I got the gun, and not thinking the chokes were much different than IC and Mod. But, there was a horsefly in that yogurt:
The proofhouse noted that the bore measurment, 9" from the breech face was 16.0mm, in both barrels. This is actually a good bit bigger than standard, 20 gauge bore. Backbored, in today speak.
Since I got the gun, I have noted that many Darne 20s are bored this way. I'm guessing it lends more actual constriction than it at first appears there is to this gun and others like it.
All of my other Darnes are much closer in an industry standard measurement, or, a bit undersize.
Not a huge fan of spreaders, and, besides, I have guns choked in a configuration more useful for earlier season use on ruffies and woodcock. But, it was eye opening out there today, to say the least.
Best,
Ted
