Centerfire rifles will punch right through 1/4" plate, even 1/2" with some .30 cals, and make it dangerous to shoot at in the future and to even handle. You're best bet is to google steel targets and get on that is properly designed to take rifle fire. That will be the hardened AR steel.
A local shop may carry the type of steel needed, but for them to fabricate it into the target you want you'll probably not save much in costs..