A full-bore (full-port) valve has an internal passage matching the pipe's inside diameter, allowing pigging, minimal pressure drop and unrestricted flow. A reduced-bore valve's passage is typically one pipe size smaller, saving significant cost, weight and operating torque where the restriction does not matter.
Inside a 6" reduced-bore ball valve is, roughly, a 4" ball — and everything scales down with it: the ball blank, the seats, the stem torque, the actuator and the invoice. A reduced-bore trunnion valve commonly costs 30–40% less than its full-bore twin and needs a smaller actuator besides. For the majority of process isolation duties, the modest extra pressure drop (the bore still exceeds most fittings' restriction) is irrelevant, and reduced bore is simply sound economics.
Full bore becomes mandatory in specific, predictable situations. Pigging: cleaning and inspection pigs cannot pass a bore step — transmission pipelines specify full-bore API 6D valves end to end, and the standard's FB/RB designations appear right in the valve marking. Flow-measurement runs: meter accuracy suffers from upstream turbulence. Hot tapping and drilling: tools must pass through the open valve. Erosive slurries: the velocity step at a reduced bore accelerates wear. Suction lines with tight NPSH margins also favour full bore's lower losses.
State FB or RB explicitly on every ball valve order — defaults differ between suppliers. VTXFLOW manufactures both patterns from 1/2" to 40", with full-bore piggable designs to API 6D for pipeline service.