Lead pellets are the traditional material for shotgun shooting, but more recently, steel has emerged as a popular alternative. Steel pellets are lighter, faster and harder, each of which offers various advantages and disadvantages. Lead has also garnered criticism for its toxicity and deleterious effect on the environment.
Steel is a much harder metal than lead, but it is also lighter. As a result, the shot string (the form a round's worth of pellets takes en route to the target) is much more focused with steel pellets than with lead. When shooting with lead pellets, each shot string spreads out much more, which means that each shot covers a larger area. This means that targets are easier to hit with lead pellets, especially a moving target.
On the other hand, the fact that steel forms a tighter shot string means it has a greater effective range. The pellets in a round of steel shot remain closer together and take many more yards to disperse to ineffective distance. Thus, steel shot is more effective at longer distances than lead shot.
Steel's lighter weight has another side effect. If an equal powder charge is applied to steel pellets and lead pellets, the steel pellets will accelerate faster. As a result, steel pellets have a greater muzzle velocity than lead pellets, generally by about 10 percent. The speed differential, combined with the fact that steel is harder than lead, means that steel penetrates more deeply than lead, and can therefore cause greater damage to the target.
Since lead is softer than steel, it can become deformed by collisions with the barrel and with other pellets that it may come in contact with. The deformation of lead pellets is one reason why lead shot spreads out faster and farther than steel shot, since deformed pellets do not fly as true as spherical pellets. The deformation can also slow down the pellets as they fly through the air, which decreases the effective range of the round.
Steel is becoming more popular in certain regions because lead is toxic. Of course, the danger is irrelevant to an animal being shot, which will end up dead either way. The danger comes in the in the pellets that fall to the ground. Fowl of various types have been known to swallow these pellets, perhaps mistaking them for digestion-aiding gravel. The birds can then digest and absorb the toxic substance and become ill or die.