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Ummmmm why don't you just buy full rifled barrel with fixed choke. Cheap enough and ad on choke tends to drop your accuracy.
I'm sure chokes are around 50 or more. And you could prob find a barrel or 100-150.
If you really want a rifled choke. Browning invector Model rifled chokes would fit mossy 500 I hear. Give your local dealer a call and find out
I heard that rifled chokes are sort of a gimmick in that the small amount of rifling doesn't do much to help accuracy.
If he's interested in spending $2+ per shell, he may as well buy a shotgun with a rifled barrel and use the expensive ammo to it's fullest potential.
The bead sight on an O/U isn't going to help much anyway...
This guy was in the military all his life and from quebec so you can't tell him what to do or even make a suggestion without being scolded. I'll find some somewhere and whether it works or not it was his idea so it will be good.