Re: Best material for gong
The gongs and other targets that 4ked Horn and I use are just pieces out of the scrap pile that didn't cost us much or was free. You will need a thicker piece if the steel is soft. I have one target that is made out of 3/8" AR500 steel and it is pretty tough. If you allow the plate to swing and use up some of the energy of the bullet, it will be easier on your target vs. having it rigidly mounted.
If you got a piece of AR500 that was 1/2" thick, I think you could shoot it with whatever uber-magnum you want and you would be just fine (it won't be cheap though). A good place to start is a 12" circle for a gong.
I keep nature balanced - I hunt everything!