308 will smoke through .250 mild steel like it wasnt even there, even past 600 yards.
If you put a chunk of AR-500 in front of the base it should help. Keep in mind how low the bottom target is. If you miss low, you can throw a bunch of dirt on the plate and they get really hard to see with a fresh coat of dirt.
You know I like my coffee sweet in the morning
and I'm crazy about my tea at night