A380 is an aluminum alloy that contains 7.5-9.5 Si, and is used for high pressure die casting. A356 is an aluminum alloy with 6.5-7.5% Si, and is usually only used with sand or gravity/low pressure die casting (permanent mold casting). Since high pressure die castings are prone to porosity formation (due to turbulent filling), they are infrequently heat treated after casting. The T6 heat treating process confers additional strength to aluminum castings, which means that A356-T6 can achieve similar or better properties to A380. The casting process will affect dimensional characteristics, so this may be why a high pressure die casting using alooy A380 was originally specified.