steel
There always seems to be differing schools of thought about shooting steel whether it's mild or varying degrees of hardness up to armor plate.
I look at it as personal risk assessment and it is up to each individual to determine how much risk they are willing to take.
I agree that steel that doesn't crater or dimple is the safest way to go. Is it the ONLY way to do it? My opinion is no but an opinion is like an anal aperture, everyone has one. I shoot handguns at mild steel as close as 7 yds.
Factors in shooting steel safely are hardness of steel, velocity and makeup of bullet, distance to target. I have made steel targets of mild steel and have had numerous thousands of rounds fired at them with no adverse effects (injury wise).
Here are a few. The 3 rectangular plates were suspended from a swing. The spring plates were shot at with a 9mm pistol cal carbine. When we realized the 2" pipe was getting dinged pretty much, we stopped the use of these carbines and magnum caliber handguns. The supports were replaced and the target faces were angled downward. I later got some 8" round 3/8" AR500 replacement plates as well. The 2 colt speed plates were purchased from MGM targets. the material is 3/8" AR500 and they reset automatically.
On another forum, the question of steel core Chinese ammo came up so I tested some of what I had ( the question was steel core ammo being AP). Left to right (bottom first) is Chinese steel core as is the one above it. Top left is German plastic core. Then Golden Tiger, another German plastic core, a Wolf. The top right made the nicest petal design. It was Egyptian lead core. This angle iron was also 1/2" thick.
Here is a club target I had the opportunity to shoot with a 8 3/8" bbl S&W500 and a Corbon round (?gr). The bullet actually stuck in the 3/8" thick plate.
This was a piece of 3/8" stainless steel with a .44 mag (same load as above) and I think a 180 gr .40