I've got 3 1/4" 12x20 AR500 plates, they've over a decade old. I typically don't shoot them under 400yds. The hardest on them is 162 A-max's from anything 280 on up I suspect due to retained velocity/energy. Even @ 600 after an afternoon of shooting they start to cup. However, as aalf stated above, just turn them around and pound the cup out the other direction.
I hang them from chains, they're fairly light and swing away pretty easily. I have one that we've shot through using the 257Wby/100TSX combo @ 300yds, so, since it already had holes, we shot it w/50BMG FMJ's @ 600yds. They didn't go through but did leave one heck of a "dent".
I've got a couple 3/8" 10" circles we use as close as 100yds as off-hand targets. I've hit them w/TSX's from 243, 270Win, 7-08, and 300Win without any issue, just leaves a copper stain. Lot's of 50 V-Max or 55 soft-point/223's @ that distance and they just blow the paint off without leaving any other marks.
