CO2 models are generally more powerful than most BB guns and are filled from a can of propellant in much the same way as a gas lighter is charged. The nozzle is usually found in the butt. To expand in the gun and propel the BB, gas requires heat. Rapid repeated fire can cause the model to freeze up and stop working until it is warmed up again. For best results keep both the model and the gas warm. Its pretty cool to get a tight cluster on a target with a smoothly operated gas model. Semi automatic models fire each time the trigger is pressed, without the need to recock the gun. Full automatic operation gives repeat fire - you'll get through a lot of BB's! Blowback is where the top slide of the gun is propelled back as the model recocks. This is a gas gun, which means it's powered by gas and doesn't need to be cocked every time. It's fast, clean, inexpensive and easily maintained; it does not require batteries to operate. Also, spring guns are the toughest, most durable of all AirSoft guns, because they're made with few breakable pieces. If you're looking for a fun, reliable firearm that'll look great and perform even better, you've come to the right place.