I've probably only ever shot about 50,000-75,000 rounds at steel, with pistols, rifles and shotguns, at ranges from 5 yards all the way out past Fort Mudge, have built a bunch of steel targets, and bought several more. Of PRIME importance is that if you weld a "foot" to a plate, be sure and weld a piece of angle iron atop the foot, to redirect splatter. Another poster mentioned concerns with pockmarks redirecting bullet material, and they do some of that, but nothing will turn a big piece of bullet material back at you like a 90 degree corner will.
Works like this: Assuming that the ammo is loaded hot enough, most bullets break into lots of fragments when they hit steel. Most of the fragments rebound very little back toward the shooter, but rather splatter 360 degrees, almost parallel to the face of the plate. Ones that go downward and hit a foot welded to the bottom of the plate will turn the corner and head right back toward the shooter. I've had to dig a couple of pretty good sized fragments out of my face, after shooting at such improperly-constructed targets. If you'll weld a little piece of angle iron onto the top of the foot, it will redirect the fragments harmlessly upward.