In this Comparison Video I go through the benefits and shortcomings of Steel BB Versus Pellet Airgun ammunition, and what they are best suited to.
Here are some basic statements that best describe Steel 4.5mm BB and Pellet Airgun ammo.
Steel 4.5mm BB's:
Steel BB's comes in 4.5mm
Steel BB's use a hard steel and are often coated in either copper or zinc.
Most BB Airguns usually do not have rifled barrels since the hard steel will not take advantage of the rifling and only wear down the rifling.
BB's are often loaded inline or one on top of each other with a spring follower that loads them into the breach of the gun. This can be done using a removable magazine or a built in loading bay.
Pellets:
Pellets come in a variety of sizes, the most common sizes are .177 and .22 caliber (.177 is very close to 4.5mm).
Pellets are most often made with lead but can be made with other light weight materials. (light weight high velocity pellets)
Most Pellet Airguns have rifled barrels since the soft lead can grab onto the rifling and create spin.
Pellets are either loaded one at a time or can be fed into a rotary magazine which then is placed into the Airgun. Pellets can not be stacked one on top of each other because they would deform and not load consistently.
Misc:
Most Airguns shoot either BB's or Pellets, a few Airguns can shoot both types ammo but compromises are made. Barrel diameter may not be ideal for Lead Pellets and the rifling can get damaged if there is a high use of Steel BB's. Some dual purpose Airguns may not even have a rifled barrel.
There are much fewer blowback Pellet Airguns since stick, or full size drop out magazines with followers work better mechanically with blowback operation.
Pros:
Steel 4.5mm BB's:
BB's tend to be a bit lower cost than Pellets.
Most BB Airguns have higher capacity magazines.
BB's because of their hardness can have better penetration characteristics than Pellets.
You may be able to reuse BB's if you have a proper backstop that can catch them safely.
Pellets:
Pellets can be more accurate than BB's since they are generally shot out of rifled barrels, especially at longer ranges.
Pellets are heavier and so will generally cary more energy and hit with more impact.
Pellets are an overall better choice than BB's for hunting since they are more accurate and will carry more energy into their target.
Pellets have the added benefit of mushrooming when they hit an object for even more stopping power.
Cons:
Steel 4.5mm BB's:
Copper BB's in my optioning can corrode over time and create a rust like looking dust which can get in your Airgun.
Steel BB's will rebound off of hard objects much easier potentially hitting the shooter.
BB's do not work well in rifled barrels and can wear down the barrels.
Pellets:
Pellets are made out lead which is considered toxic.
Pellets can deform pretty easy so care is required when handling.
Pellets may not penetrate harder objects as well as hard Steel BB's.