138mm Inner Barrel for GBB Replicas (6.02mm – Tightbore) MAPLE LEAF GBB Inner Barrels have been designed to improve the performance of your Gas-Blow-Back (GBB) Airsoft Replica. Many OEM Airsoft Replicas feature lower-tolerance Inner Barrels with inner diameters as high as 6.08 or 6.10mm. By installing an Inner Barrel with a reduced inner diameter, less air can escape around the edge of the BB whilst traveling down the inner barrel. This escaping air can destabilize the flight of your BB, so a tighter inner barrel results in improved flight.