gas blow back guns are one of the most powerful and realistic types of airsoft guns due to their use of pressurized gasses. With a pull of the trigger a small burst of gas is released that helps propel BBs out of the barrel. From that small burst gas guns average FPS ranges are quite high often ranging 300 to 400 FPS and above with leftover energy to spare. The leftover energy can be felt as what is known as blowbacks giving a similar experience of recoil you will with real firearms. For some models of gas guns a portion of those energies are redirected into a mechanism that helps cycle the gun in a realistic fashion such as pushing the slide on pistols or the bolt on rifles. The most common gases used for airsoft are CO2 cartridges or Green Gas (a special blend of propane gasses mixed with silicon oil).