dla,
I have some 3/8" T1 targets and they work fine for pistol calibers. My friend's 454 Casull won't scratch, bend, or dimple the stuff (even at very close range). T1 is very tough stuff.
However, .223 rounds out of my AR-15 tear it up something fierce. At 50 yards, 55gr soft point bullets penetrate the steel after repeated hits. I recommend 1/2" or even 3/4" T1 for rifle fire inside of 100 yards.
Dan