Steel shot 20 gauge ammo gives you options in places where lead isn’t allowed by law. Due to the accidental ingestion of lead shot by certain migratory game bird species and the subsequent lead poisoning it allegedly causes, the U.S. Federal Government has prohibited the use of lead shot when hunting waterfowl in some areas. Consequently, the use of steel shot in the 20 gauge as a replacement for lead shot has gained popularity.
It should be noted that because steel shot is significantly harder than lead shot, it should only be fired in shotgun barrels specifically designed to handle steel shot. In addition, because steel shot does not deform like lead shot does, you might consider using one size larger choke than you would normally use with lead shot. Finally, when choosing 20 gauge steel shot shells, you should choose at least one shot size larger than you normally would when choosing lead shot because steel shot is significantly lighter than lead shot and thus, it retains less downrange energy.