I have conducted similar tests on abandoned/ wrecked cars, and the best penetrating handgun rounds were the old "Geco-BAT" copper bullets, which are no longer made. Too bad, they had excellent expansion properties in flesh, and amazing hard surface penetration. Testing them on an abandoned 1960s era Fury 3 (car), side to side with windows rolled down, the BATS went thru the car completely.. that's two doors consisting of heavy sheet metal, tempered auto glass, door internals (?) and the inner door liner! Even 9mm, 124 grain NATO ball could not do this, until I fired some thru a Colt 9mm CAR 15 :uhyeah: My handloads with this NATO bullet group 1 inch, (3 shot group) at 50 yards in that Colt.
I was interested to see the 45-70 515 lead bullet results in the Box-O-Truth tests. I did not expect it to exceed .308 ball results, but it did. Interesting because my handloaded, hardcast .662 round ball 12 GA slugs, (weighing + - 420 grains) do not exceed the hard sheet metal penetration of SS 109 ball ammo. Of course that SS109/M855 is pretty impressive ammo, steel core and all that.
I would like to see the following tests, may do the exact same tests myself sometime..
SS109 penetrators (.223)
7.62 x 54 Silvertip steel core ammo vs heavy bullet copper/lead ball
7.62 x 39 steel core Chinese ammo, the copper washed stuff
.308 AP ammo.
30-06 black tip AP
.357 Sig
10 MM auto
.475 Wildey Survivor and .45 Wildey Survivor
.44 Magnum