I have one of the 45-410 10 inch Contender barrels with the real iron sights - no vent rib, also one of the old octagon 45-410 10 inch barrels.
In the first barrel I have shot factory 45 loads, I could hit the paper plate at 50 yds but that was using all of the paper plate. This was using Federal cast lead hollow points and Remington UMC (Yellow Box) lead flat points - both shot about the same as far as accuracy. In my opinion accuracy was a bit lacking, although I did not experiment much to find a load that shot better. I had always heard the same as you - big jump to the rifling will screw up accuracy.
In 410 mode the gun was fun on doves and black birds, even got 1 pheasant. I used the octagon barrel a few times at a friend's house to shoot trap - scores were not impressive, but it was good fun and sparked a lot of conversation with some of the other shooters present.
On the 410 buckshot. While I lived in Bolivia I had one of the Rexio (Super Comanche) break open single shot pistols chambered for 410 - I imagine most of the guys on this forum know what that is as I have seen them around in the US for the past 3 years - they are cheap (made in Argentina), and in Bolivia it was the closest thing I could find to a Contender. It had a 10 inch barrel and a bead front sight. I used it as a sidearm while hunting in Bolivia, I actually took 2 deer (they are much smaller than white tail deer) with it using the 410 buck shot - 4 pellets. Both were at relatively close range 20-25 yds. I also took many of the local partridge, a few doves, a couple of armadillos, rabbits, an alligator, and a duck or 2 with that pistol - it was great fun.