targets
All good answers.
I try to steer people to 3/8" AR500 for all targets which should take anything up to rifle rounds except magnums where I would suggest 1/2" thick.
For pistol only, AR400 will handle everything including magnum rounds.
Mild steel will dimple even with handgun rounds but not enough to be a major problem if you face the edges off the dimples. Rifle rounds will CRATER mild steel and is asking for splashback, ricochet, pieces of all sizes coming back. Just not a good idea. If you have a few hundred yards, mild could be used as sacrificial one time use but just not a good idea.
This is the kind of dimpling you can expect on regular mild steel with pistol calibers. There may have been a few .357 mags on there.
This is a spring plate I fabbed from a 3/8" mild steel face and 2" pipe inside a dumbwaiter spring. Notice the angle and splash residue.
This piece of mild steel was hit by so many thousands of pistol rounds that the plate started to bend.
This shows a combination of 3 handgun rounds and the rest rifle hits on 1/2" mild steel.
A 9mm, a .40 and a .44mag are in there.
This was a test plate I made just for display.
If a rifle bullet hits a crater or if a bullet hits half a crater, a portion or major portion can come right back at you and cause severe injury. Just don't do it.
AR500 (abrasion resistant) is the way to go. I bought a few self resetting colt speed plates from MGM target. they reset faster than you can hit them and will last a really long time.
I got 5 of these 3/8" by 8" rounds from a guy on this site (mcosman) for about $25 ea (probly more now) but they are AR500. Wideners also has some AR500 faceplates that you can fab stands for.
Good luck.