Agreed, 5.56x45mm will typically make mince meat of 1/4 inch mild steel plate but will have trouble even making a dent in a high hardness steel of "armour" grade.
Can't wait to try the 300WinMag loads again soon, just wish I had a 300RUM to really spank them out.
Core construction matters big time. The older 163grn pills are of better construction, probably steel/tungsten alloy because after passing through my 1 1/4 inch steel plate they are still intact and sharp.
Compare to some 150grn steel core rounds I've shot out of the 300WinMag and they fail miserably, the cores are apparently melting/deforming upon impact with the steel. 3400fps with the 150grn pills is not managing to defeat the target steel where 3200fps with the 163grn pills is working.