Lambda Japan added more to their Smart Barrel Inner Barrel series. The Smart Barrel VSR NB is made of Carbon Steel and  they cut the bridge for the custom barrel due to requests from their European customers... "This series is 6.03mm Inner Dia. This shape is customer's request. Many customers asked to us so we started selling as regular lined up. SMART BARREL is lower cost model but keep quality. Made of Steel and process the plating so done rust-preventive measures."