There are CO2 air guns using the small 12gram CO2 cartridges, some using a larger 80gram cartridge, and the fixed CO2 tank. Others use a refillable air tank (PCP, or Pre Charged Pnuematic) that is filled via compressor or scuba tank.
Calibers run from the common .177 and .22 on up through 9mm and .357, on up to .50 caliber. Velocities allow them to take down deer-size game. Definitely dangerous weapons, but somewhat cumbersome and slow to load. Some are magazine-fed, allowing several shots before reloading. Effective range remains well below that of 'powder-burners' though. Costs are extreme for the true hunting grade air weapons.