The same example of steel shot damage gets brought out time and time again, then there is a "mate" whos a gunsmith who sees damage all the time. There is no solid evidence being shown about the damage.
Hundreds ( if not thousands ) of fowlers use steel in full chokes, and through older guns, with no damage. It's up to the individual to choose. Too much scaremongering.
One of these days I'm going to fire some steel through a full choked non steel proof gun, measuring the bore at the muzzle before and after say 10 shots. I'll measure it down to the 10s of microns just to be accurate.
Then i'll fire steel shot through it without a enclosing wad, just to see what really happens.
If any one wants to donate a cheap 3" single barrel then feel free!