BB Guns vs Pellet Guns

Not Airsoft:
First off we are not talking about 6mm Plastic Airsoft Guns but rather 4.5mm Steel BB versus .177 caliber Pellet.

Same but Different:
Generally speaking a BB CO2 Pistol and a Pellet CO2 Pistol are very similar, they just shoot a bit different types of ammo, that will determine a couple mechanical differences in both guns. You may come across a CO2 Pistol that can shoot both Pellets and BB's but these are not as common.

Magazine Types:
The fist main difference is usually the magazine design, since lead pellets don’t stack on top of each other very well compared to steel BB’s, action CO2 BB and Pellet pistols will have a very different types of magazine designs. Pellet CO2 Pistols will generally use a rotary type magazine while BB Pistols will use a stick or stacked ammo magazine. In terms of realism, a stick or stacked magazine is better suited than a rotary magazine. Some Action Pellet Pistols will have what looks like a stick magazine, but the stick will actually have two rotary magazine on either side. You will often find higher capacity magazines in BB CO2 Pistols since Rotary magazines usually only offer around 8 rounds (16 on a double sided stick style).

Read full article here:
https://www.replicaairguns.com/2015/2/20/bb-guns-vs-pellet-guns.html