I built some gong targets from mild steel. The plate is 1/2" thick.
I do not recommend shooting at steel closer than 15 yards. I regularly shoot at mine at 25 and 50 yards with lead handgun bullets. Up to 44 Mag.
The closer you get, the more the chance of lead splatter hitting you. The lead splatter that does reach you at 25 yards is basically like tin foil, and was floating in the wind. It really bothers shooters around me and they cry and moan when they feel this stuff touch their clothing.
Go ahead and shoot it with a rifle, just to see. After you see what looks like drill holes all the way through it, you will either stop, or you will shoot it till it looks like Swiss cheese and have to replace them.
A high power rifle bullet will shoot through an amazing thickness of mild steel.
A friend of mine ran a rifle range. He tried steel targets, none would last. He put up armored personnel carrier covers up at 200 and 300 yards, and the rifle shooters shot them to absolute pieces. He finally settled on engine blocks. They were cheap to replace.