When does a CO2 gun make sense?

A CO2 gun makes sense for casual plinking, backyard target shooting, and training, especially when you want a realistic feel of a semi-auto pistol or rifle.

They are inexpensive to operate and require no pumping or filling beyond installing a 12-gram cartridge. The catch is that CO2 is temperature-sensitive: performance falls off in cold weather and improves in heat, and rapid shooting cools the gun and drops velocity. They are not the right tool for serious hunting at distance.