Cool test. But I think unless the steel is fixed, it can skew the results a bit.
I was not able to get through a 3/4" mild steel plate at 40 yds with 5.56. (16" 1-9 twist, barrel M4gery) Not Lake City, AE 64gr tracers... 55gr FMJ, 62gr HP... nothing.
Heck, not even a .308 or 54R round went through it. Where from your tests, you would have been through it no problem.
But it was suspended by chains and could swing upon impact. This is the wild card I think.
I'll bet money the 54R would have went through if it was bolted into place. It almost did even though it was hanging.
All rounds fired at the plate, (And there were a lot) deformed the back badly. But yet another one for the caliber crowd.
7.62x39mm did NOT do as much damage to the plate as 5.56 did.
It was definitely a wider crater. Just nowhere near as deep.
Out of all my unofficial "tests" with 5.56/.223. The American Eagle 64gr tracers went the deepest. The also lit it on fire!
I guess this goes to show that until you go out and ACTUALLY shoot some things. Don't believe everything you read on the internet.