A combustion system including a transition piece and method of forming the transition piece using a cast nickel-based superalloy is provided. The transition piece includes a body defining a flowpath and enclosure, the body having a circular inlet section for receiving combustion product from the combustor and an outlet end for flowing the combustor products into a first stage nozzle of a gas turbine. The transition piece comprises a one-piece single casting construction of the circular inlet section, the body, and the outlet end. The transition piece is formed from a nickel-based superalloy. The transition piece has a thick-to-thin ratio of wall thickness of thick transition to wall thickness of thin transition of about 1.1 to about 2.5.