Buy the right steel, an Abrasion Resistant hardface steel like Ar-400 or AR-500. Hardening carbon steel will probably lead to a shatter or it won't be hard enough. As it is now your material won't get holed by the pistol rounds but it will get pocked and cratered. Uneven surfaces send things back in the wrong direction. If you have good flat steel 10 yards is plenty safe. If you're shooting at dumb stuff like cratered steel or old car rims, then nothing is safe. I've heard things go over my head after returning from the 100 yd line steel that idiots holed with rifles.