Due to the toxicity of lead to almost all forms of life, and the usual junk science, governments across North America were conned into believing that ducks and wetland creatures were being poisoned by ingesting lead pellets that fell in water. So, laws were passed that banned the use of lead shot for hunting migratory birds or any game that would result in the pellets falling in the water.
Up to that point, lead was the only material used for making shot (not counting experimentation or specialty applications, which combined wouldn't have amounted to even a fraction of 1% of the shot produced). Taking into consideration issues such as toxicity and cost, steel became the material most often used for "non-toxic" shot.
There are two primary differences between lead and steel shot - lead shot is heavier by volume, and lead is very soft when compared to steel. Barrel steel always had to be hard enough to retain it's shape over decades of use, but elastic enough that sudden increases in pressure (when a shell goes off) wouldn't cause it to split as it would have were the steel too brittle. The lead shot was, and remains, soft enough that it would deform before the barrel steel. Steel shot is significantly harder to the point where, if a tightly packed shot stream passes through a tight choke (say "full") in a gun made from the elastic barrel steel, the barrel will bulge before the shot deforms. Usually it will take several steel loads through a tight choke to cause visible or catastrophic damage (but don't count on it).
For example, the first shot stretches the barrel imperceptibly, at the point where the shot starts to enter the choke constriction. The second shot stretches the barrel a bit more. By the third shot, a bulge is beginning to be noticeable, if you were looking closely. The fourth shot, even more so. By the fifth shot, the shot stream is not only slamming into the choke at the end of the barrel, but it's hitting the ridge caused by the bulge and even higher amounts of energy are transferring to the barrel (not to mention the havoc this is causing with shot patterns). A few more shots and the barrel will be badly bulged or split wide open at the muzzle.
It doesn't always happen that fast. Some barrels are harder than others, so there's no hard and fast guidelines to this. It's not unusual for it to take a season or two before the barrel is ruined.
As it turns out, steel shot doesn't require as much choke constriction to produce the same pattern at the same distance. So one option is to open up the choke(s) to "modified" or more open than that for closer shots. This allows for the use of steel shot shells through guns that were never designed for it. This is feasible with most shotguns made from about 1910 onward.
To do this safely, though, you need to know the actual amount of constriction as compared to the nominal bore - at a minimum after the reaming is complete, if not before starting, too. Simply knowing that a dime will fit through the choke doesn't tell you how much diameter is lost going through it. Two guns could have a muzzle opening that fits a dime exactly. But, if one has a larger nominal bore, the blocking effect on a shot stream of steel shot passing through it could be disastrous, whereas the other may be just fine because the shot stream was never expanded as wide while going down the barrel to begin with.
