Heat is unevenly distributed into the steel due to the chamber filling pattern. Where the most heat is input, additional cooling is needed. Large temperature differences arise, and these temperature differences over the length of the chamber lead to significant distortion. The piston, which has been machined to fit the chamber exactly, will be unevenly worn as it moves through the chamber.
That is why it is very important that during the design of the cooling of the die, the shot chamber is also considered.