"Gong" targets are great, but not necessarily for usage within 50 yards. Mild steel will work, but most center-fire rifle rounds will go through it, unless it is excessively thick.
"Planted" steel plates won't last very long, for you'll put a lot of holes through them. If they're "swinging targets" (mounted, say, between two trees and affixed to steel chain or cable), the initial strike of the rifle rounds will be lessened slightly, due to the steel plate reacting to the sudden impact. To a certain extent, welding steel plates onto spring steel coils (car springs) will lessen the impact, but they'll also get knocked over after one solid strike....if they're not solidly implanted in the ground.
The "gong"-type steel targets are better suited for .22 and center-fire handgun calibers. I've been using a 1/2" piece of steel plate (12" x 12") for about 10 years. The "face" of the steel plate gets "dimpled", but nothing has gone through it so far. When the "face" becomes too dimpled, it gets turned around, with a fresh coat of white spray paint, and the "dimples" will eventually be straightened out!
The HARDENED/TEMPERED steel plates might cause you some problems with center-fire rifle rounds! I tried out an old "manhole cover" that was hardened steel, and even .30'06 AP rounds didn't go through it! Even at 100 yards, however, I was worried about richochets and bounce-backs! Even if I had angled that manhole cover, so that the rounds would be directed downward after impact, I didn't fully trust the "rocks in the ground" possibility!
By the way, I have a "lucky piece" that I carry in my watch pocket. It's the hardened steel penetrator from a .30'06 AP round that was fired at a 3/4" thick mild steel plate at 200 yards. The copper jacket and lead from that round were fully stripped from the penetrator upon impact, but the the hardened steel penetrator penetrated THROUGH the steel plate! Actually, the tip of the penetrator was exposed through the rear side of that steel plate. It took a LOT of hammering to free the penetrator from the plate, but the sharp point of it was unscathed by the hammering! It's about the size of a .223 bullet, and if you didn't know that it had been fired, you'd think that I had picked it out of a box of UN-fired penetrator projectiles! HMM! Maybe that's why they call them "armor piercing"?
LEAD bullet vs. STEEL plates is okay. Metal jacketted bullets vs. STEEL plates are PROBABLY okay, but AP rounds vs. STEEL plates is definitely risky business....especially within less than 100 yards!