I use a lot of ar steel but regular mild steel will work fine after a certian distance. I have used half inch mild steel at 500 yards and it holds up fine to my 300 win mag just fine!
I gave a remington 700 5r milspec 300 win mag for sale if interested pm me for details. Have plenty of extras brass bullets etc. If you want. I take this rifle to 1000 yards at ease with my hand loads.