I've shot enough American steel loads through my old BSS to shoot it off face on the right barrel, which always fires first. I shimmed the hook and keep using it. Before starting the left choke was opened to match the right, at .018". Never a sign of any damage. Going off face was just a combination of heavy loads being fired through it for a long time, not the fact that they were steel shot loads.
The duck hunting author, Worth Matthewson, from the Pacific Northwest, wrote in one of his books that he shot off the shelf steel loads for years in an old LC Smith. He took it to a gunsmith for some issue and learned that the chokes were full and full. There was no apparent damage to the gun.
I am careful about not using steel in really tight chokes, but have no qualms about up to .018".
