a 270 winchester, 30-06, 375 h&h, 300 win, and 338 win with normal lead tip hunting bullets will penetrate 3/8" - 1/2" steel from 100 yards at anything close to a 90 degree angle like a hot knife through butter (seen with my own eyes). At a 45 degree angle, it will dent it pretty good and not take too long to start creating holes. I would think a 3/4" piece of steel at a 45 degree angle would probably hold up, but I would go for 1" thick my self.
and of coarse, like you stated, angle it towards the ground.
Just FYI, I had a 375 h&h bullet hit the edge of a 1/4" thick plate which cause it to keyhole through the 1/2" steel plate behind it. couldn't believe the bullet went right through it while traveling sideways.
