Generally speaking, old shotguns (“old” being a bit of a nebulous term we’ll try to define in a minute) were built to shoot lead shot, not the much harder steel shot popular for certain types of hunting and even for some target shooting today. The rule of thumb has always been to just not shoot steel out of your old gun, but how bad is it, really, to do so?
Of course, it depends. Shooting steel out of a barrel that wasn’t designed to handle it can cause a ring bulge on the barrel just behind the choke or even “irreversible damage or harm to the shooter depending on the firearm,” according to Browning’s website. They list a set of guidelines for which steel loads can be used in which Browning models, and you should see if your gun manufacturer has a similar list. The ring bulge is a cosmetic problem, of course, but it can also create a safety issue if you continue to shoot the gun, as a problem in the barrel is certainly not going to get better. In old side-by-side or over-and-under guns, it can eventually cause problems with the solder that holds the barrels together.
A major part of the problem with old shotguns and steel shot is the choke. Steel through a full-choked gun just doesn’t work well, resulting in blown patterns and that ring bulge, which is a result of the shot literally pounding out the barrel in the area where it begins to constrict. It seems unclear if this presents a serious safety issue or not, but regardless, you probably don’t want to damage your gun this way and take a risk of physical harm to yourself.
Many older guns have fixed chokes, and if they’re fixed full, you should avoid steel completely. Some research out of England, where non-toxic shot is the rule, not the exception, indicates that regular steel shot out of older barrels is safe as long as the gun uses a Modified choke or something more open than Mod. This applies only to standard steel; loads larger than No. 4, or with pressures in excess of 10,733 psi, or with velocity higher than 1,400 fps in a 1-ounce load or 1,250 fps in a 1 1/8-ounce load are considered “high-performance” steel loads and should not be used in a shotgun that’s not specifically marked as proofed for steel shot.
In the U.S., any gun made since the late 1980s is in the clear for steel, as they were made with steel in mind. Older than that, and you’ll want to verify with the manufacturer or a gunsmith whether your barrels are proofed for steel shot before you go throwing high-performance steel down the barrel. If you cannot verify this, you should be OK to shoot standard steel loads as described above (small shot size, not too much velocity) out of a gun that’s choked Modified or something more open.
Personally, I don’t fool with steel out of an old gun. There are too many factors and a whole lot of “ifs” at play, and I have modern guns I can shoot steel out of when I am hunting waterfowl. If you really want to shoot steel loads out of an older shotgun, you should be sure you know the choke constriction, be careful with your ammo selection, and consider checking with a gunsmith to get a professional opinion. Alternatively, you can shoot bismuth loads out of an old gun safely, because bismuth is softer than steel and won’t damage old barrels. And it’s still non-toxic, so it’s legal in areas where lead shot is banned.