It's 60kg per meter with an 8mm web - the .204 24gn at 4400fps goes through it at 100m, but I don't think it would do it much further than that. I even put a 180gn 8x57mm cast lead bullet through it at 50m, but at 2500fps. When I've dug bullets out of the butt behind it, I've found some steel discs that have been punched out of it.
I haven't tried to penetrate the 13mm flanges as yet but I guess I'll have to try some day. I know that corrugated steel roof sheet stops 710fps CCI Quiet past 165m.
5mm mild steel withstands .22LR at any range, but you have me curious now. I'll have a look around to see if I can find some various thicknesses of mild steel to test penetration range on.
Shooting 4" targets at 200m with irons sounds like a fun challenge