BavTac LPV TM MP7 GBBBavTac Low Power Valve for the Tokyo Marui MP7 Gbbr. There are a few things that affect FPS when it comes to Gas rifles. The type of gas used. Different types of gas have different expansion rates. Airsoft rifles are typically designed to use a specific type of gas. Using a different type of gas can affect the FPS. The length of the barrel. The longer the barrel, the more time the gas has to expand. The weight of the projectile. A heavier projectile