Mossberg makes Turkey versions of this gun with choked 20" and 22" barrels. You would probably need to install a more open choke to shoot slugs or steel shot. All the Turkey models are camo. You would have to contact Mossberg to find out availability of the barrel alone and perhaps in black only. I would keep the 28" as-is because it will save you from having to get one later should you choose to go skeeting or fowling. I get $60 to cut a barrel and re-install the bead sight (no, I'm not soliciting for business), and others have mentioned the going rate to thread for choke is about $100, not inc. chokes. You can probably find a new/used camo. barrel for that money and just paint it black. Be aware that some barrels are overbored before the choke and can not be re-choked as there is not enough metal.
If WM messed-up, which serial number was recorded? I would hope the serial numbers are correct because if the 500 you bought is found at a crime scene, police will be at your door asking questions...