There are many silver soldered barreled side by side and over and under shotguns that should not be fired with steel shot. I have a Churchill Windsor 10 Gauge that is choked full / full and does not have the screw in chokes. This gun, like many, was designed in the 50's and 60's, and produced well into the 80's without any design modifications. Firing heavy charges of steel shot can cause the barrels to bulge before the choke, (about 3 inches from the muzzle), and the barrels to separate.
When the first steel shot shotshells were designed, they did not have the development the newer shells have that are currently on the market. Many were just lead loads with the lead shot replaced with steel, and the powder charges adjusted accordingly. These offered little to no barrel protection, and as a result many older shotguns were damaged by these loads. While the newer steel and tungsten loads are much better, there are still a large number of older shotguns out there that should not fire these rounds.
For these older guns Bismuth loads are safe to fire. Bismuth is a non toxic metal that is very soft, and will pass through the chokes of these older weapons without causing any damage. The downside to these loads is they are very expensive, costing over $3.00 a round for some of them. But if you have a nice older shotgun it's a small price to pay to avoid possible damage.