Full bore ball valve is a valve that the hole diameter of its ball is the same size with the pipe size. Reduced bore ball valves is a valve that the hole diameter of its ball is not the same size with the pipe size. In minimum the reduced ball valves ball diameter are one size lower than the pipe size i.e. 4 inch pipe and ball diameter is 3 inch (usually symbolized as 4 x 3 inch ball valves). From its definition we can quickly know that the full bore will have less pressure drop and reduced bore will have more pressure drop since reduced bore is just like a restriction orifice that narrowing at the middle part.