This is sort of related to Ken61's non-tox post. I asked a ACGC member gunsmith recently about whether installing steel-proof chokes on a vintage gun would allow the gun to shoot steel shot safely. I thought his response would be of interest here:
I personally do not subscribe to the idea that changing a guns choke makes it steel shot worthy. Older guns were designed around an age of paper shotshells and felt wads with little or no wad cups to speak of. The forcing cones on these guns are short and fast in order to seal the gap with the felt wads. Now if you were to fire steel shot through this same barrel the steel shot column is contained in a modern wad however the steel shot must now arrange itself in the column as the steel shot does not compress and deform like lead does. This is why most steel shot proof guns, like all new AYA’s, have at the very least long forcing cones which allows the shot column to elongate so that the steel shot to find its place without scoring the bore. Most modern guns designed around using steel shot have long cones and are over bored for this reason. In my day of running our hone I saw many bores that had long grooves or scoring from steel shot use and it would be quite apparent after honing. This would start at the forcing cone and taper off about half way down the bore
