I use 5/8" plain old A36 steel for a gong. When it finally gets too many chunks out of it I cut another one. I don't really shoot them up clsoe but at about 300 yards works great up to 300 win mag. It's cheap enough you can make several for the price of some of the other steels mentioned. I just pick up the drops out of the shop.
A hanging gong will not have to be as thick as a standing target that can't swing.
