Been there. Depends on the thickness of the AR plate. If you want to shoot steel at 100 yards, your targets are going to be in the 1/2" range, and be HEAVY. Thinner steel of course for shooting at longer distances.
I've still got a set of 1/2 scale Combat Silhouettes made from only 1/4" T-1 plate that I've used with my handguns, primarily .45's since the early 80's, and they're still going strong, but looking purty weeblie after thousands of hits.
Not a good photo of the Combat Silhouettes, but you get the idea.
A 204 at 100 yards would sail right through them though, so choose wisely.