I have all AR500 1/4" gongs in 12, 8, 6, 4, & 3 inch. They ring with 22lr out to 150 yards and beyond! They hold up to normal handgun from point blank on out. Mega handgun and rifles are fine from 50 yards on out.
The more energy you put at them the more they dance, that is one reason for thicker plates.
I have a 40 year old 1/4" silhouette (1/3 size man) of unknown hardness, assume AR500 or close, it has a big dent in it from a 50 cal. lead round ball from a flint lock rifle at 20 yards. (2inch dent, one inch deep!)
One of my 12" gongs has a minor crater, from 308 ball ammo at 10 yards. (dime size shallow crater)
You take care of your plates and all is well, but you never know what your friends will do to your steel!
Ivan