T-1 steel is good if you can get it, and for calibers heavier than .22 LR, I would want a thickness greater than 1/4" . Here's a bit of caution FWIW: In the distant past, I was the IHMSA State Director for N.C., and traveled and shot regularly throughout the state and surrounding states. I've seen many hundreds of thousands of rounds of handgun ammo fired at steel in IHMSA and NRA matches, generally without incident.
HOWEVER, stuff can happen: In a Hunter's Pistol match about 20 years ago, a friend shooting near me had a hunk of jacket from a .357 JHP bounce back from a 25 yard chicken and severely cut his lip. In a different match, I had all or part of a 158 gr lead SWC bounce back from a 25 yd. chicken or rail and strike me on the forehead above my right eye, nearly knocking me out.
Not related to these incidents, but responding to complaints and requests from member clubs, the NRA later changed the distance of the nearest target- the chickens- to 40 yards, rather than 25.
I realize that many speed and cowboy matches shoot steel closer than 25 yards, mostly with targets angled slightly toward the ground, to deflect bullet splatter. I like to look closely at such targets, with a dubious eye. Good strong shooting glasses are a must, in any event.
