I have a American Arms 32 ga. O/U and when I bought it around 1993 you could buy Fiocchi 2 1/2 shells by the case for about $ 5 a box. I bought several cases and have molstly shot skeet and a few doves with them. I made a die set to fit my MEC reloader and have loaded my own with card and fiber wads shooting 5/8 oz. of shot.
Shotgun barrels are reamed to the finished bore size, I think if you could find someone to ream your 410 to 1/2" ( .500 ) bore size and bore or ream the chamber and forcing cone , it could be done.
To pay a gunsmith to do it ? I think it would cost alot, but if you could find someone with a lathe and the ability to make there own tooling it might be a fun project.
My O/U doesnt get much use anymore and I dont know if it has become anymore valuable over the past 16 years, so I have thought someday of converting it to a double rifle as I have already built one on a SxS .
There are alot of people who dont know there are 14 ga.s 24 ga.s 32 ga.s and a lot of oddballs out there , but myself being a wildcatter and one who is ALWAYS thinking of guns and " what if " I had to pick it up ( the 32 ga. ) when I saw they were available. Jedman