same thing here, i was using mild steel at 385M / 421Y, cratered with .3006, dented a bit with .308, a small pock mark to near nothing with .223.
had some guy with a mosin and steel core ammo drill holes right through it, target didn't even move. looks like swiss cheese now.
but on the AR400 i had gotten donated, that steel core didn't leave much of a mark on it at all. but i'm still able to use the other non swiss cheese mild steel targets for at least another season. AR (400 or 500) is definitely worth the investment if you shoot alot.
also as commented above, conveyor belt is the way to go if you can get it. mending that chain gets old quick. i think JC offers conveyor strips as a stand alone product, i got a whole bunch off him "mounting kits" strips are 15" long and 1.5 to 2 inches wide. they take a whole lotta hits.
if you can find some belt, i've found the easiest (but stinky) way of cutting it is an hand angle grinder with a metal cutting disc, cut about 3/4" through then slice the rest with a utility knife.
i got the JC double T posts too, but not sure if i can actually utilize them in my steel match like i thought i was going to. but for casual individual shooting those T posts are handy. if you have smaller targets, you can fit 3-4 on them if you don't mind drilling an extra hole or two.
http://www.snipershide.com/shooting...et-handing-method-jc-steel-targets%2A%2A.html