RPB means RTI Proprietary Barrel. The RPB barrel was specifically designed by Lothar Walther to match perfectly with the RTI Prophet II. Both the standard and performance versions carry the 600mm barrel, which has a 1:16” twist rate for the 5,5mm (.22) and 1:20” twist rate for the 6,35mm (.25) barrel. The RPB barrel has a light choke and shallow grooving, making it a very good barrel for both slugs and pellets. 4,5mm (.177) and 7,62mm (.30) versions come with the original Lothar Walther barrels. 510mm versions of the RPB Barrels are also available. Of the Compact versions, only the 6,35mm (.25) comes with a 500mm RPB Barrel.