Do yourself a favor and get yourself a good AR500 plate. You can build your own frame if you need to, but a good plate will last a long time. Mine says impact velocity must be less than 3200fps. So far I've shot it with various 223 rounds, 7.62x39, 270, 25-06, 9mm, 40S&W, etc. The only rounds that have left any dimples are the 180gr soft points at 2600fps from my 7.62x54R Mosin Nagant, have not shot it closer than 150 yards, very weird. Everything else just splashes. Here's mine.
300 yards, 3-shots with my 223.