Yes, 24" and no choke means chop-chop. A 12 ga. shell will fit somewhat loosely in a 10 gauge chamber. Have you tried a 10 ga. shell? A modern one won't go in all the way because your gun would most likely have 2 7/8" chambers. If you measurements are accurate, .771" at the muzzles would mean your barrels are at least .043" over nominal bore size for a 12 gauge. This would mean that someone would have had to hone over .021" out of your barrel walls, which might make them dangerously thin, and further reduce value. You did mention in your for sale ad that the barrels are bright and shiny, so I guess it's possible someone honed the crap out of them to remove deep pitting. Hard to tell with the information we have.
I assumed the setscrew you mentioned was a retaining screw for the hinge pin you called a side screw. I don't know for certain because I don't have any experience with Colt doubles. I merely kick myself from time to time for letting a very nice 1883 hammerless Colt double get away from me at a local auction about 10 years ago. It kinda sounds to me as if you have little more than a parts gun here. As such, the value would depend on how many good parts can be salvaged, and then finding a buyer for them. If the barrels aren't honed thin, someone might want it for a Cowboy Action gun.
