I shot through 3/8" steel (44 mag pistol target) with regular old 55 grain fmj's out of my AR15 A2 (20"), and it barely swung the target on it's way through. It moved it so little that I thought I was just nicking it, so I shot at it again then checked the target. Two holes were drilled right through the middle - and that was from a range of 75 yards. Which reminds me - I still owe my cousin a 44 mag target.

I don't know what 'welding steel' is, but I'm guessing it's not armor plate.
I had several boxes of green tips too, and I unloaded them on a huge chunk of granite in Michigan. Normally I wouldn't shoot at a rock, but you'd have to see the location, and believe me when I tell you nothing was coming back from where that boulder was sitting. But I digress. Those green tips made a bunch of little rocks out of one big one. Every time I hit it, it knocked a chunk of granite off about the size of my fist.
5.56 will penetrate a lot more than most folks think. If you're in a car, and someone is shooting at you with a 5.56, you might as well be wrapped in paper mache.