Thickness of the plates may also be critical. I bought some knock down plates cheap on ebay. Seller claimed they were AR400 1/4". 9mm 124gr FMJ at 1070fps made small dents in them. Maybe they aren't really AR400. I have no way of knowing. Maybe 1/4" is just too thin. I suspect they're fine for .22lr...but not great for larger stuff.
Have any of you tried to simulate a plate rack with a setup of 6 of these and 6 shepherd's hooks?