Just a bit of an enquirey?

How do the firms that rust blue comercially polish their barrels initially??
I mean before they blue them
The only way I know is by hand, IE 600 wet and dry 1200 wet and dry and down as fine as crocus cloth
Surely?? these shops don`t do it by hand, too labour intensive??
Do they use some sort of soft wire wheel??
I am aware of the wheels that Brownells have

Cheers