Steel use flowchart
1/4" AR has to be pushed back to about 500 yards to hold up to 308, elsewise it will dent. I build steel targets for fun and profit, and here's my write up on shooting 1/4" AR400 http://moatargets.com/blog/14-ar400-as-a-long-range-rifle-target/ . AR500 would act about the same, the hardness (400 vs 500) only really helps with preventing pitting from velocity, not denting from total energy, based on my testing to date.
For long range flashers, consider something like the Target Impact Signal Systems (TISS) units that have a sensor on the back of the target and a camera flash stationed nearby to signal hits.
It's kinda expensive, but the Mozambique and Mini MO units that I build are designed to be used as long range targets that are easily checked for hit/nohit from the line. https://www.moatargets.com/moa-targets-mini-mo-ar500-steel-reactive-target
I'm headed out on a three week shooting road trip. If you've got any questions, please drop me a line at mitch@moatargets.com