I'm currently the President of the El Paso IDPA and Multigun, so let me share what I have seen when shooting our various steel targets. We have everything from locally built steel targets of an unknown source, mild "A36" steel targets, Pistol rated MGM targets, Budget AR500 targets and high-$$$ MGM AR550 steel targets. We hold matches twice a month (at least since 2010, but likely for years before that) and always use steel targets. We can have as few and 10 shooters, and up to about 30 shooters.
Mild unknown steel/A36 Steel targets: This steel has been shot constantly with 9mm/40/45, as well as some 9mm major, with little dimpling or deformation, as long as it is 3/8th inches thick. One time we accidentally bought some 1/4" thick targets and they will dent with 9mm, so they are shotgun only. We will shoot this steel from 10 yards away and no one has even been hit with anything more than the small amount of spall that is common to all steel targets. It will deform heavily if shot with a shotgun slug and be ruined. Rifle bullets will make deep divots, but generally won't give a clean pass through. Some of these steel plates are likely 15 years old, as the last two club presidents did very little to buy new equipment for the club.
Pistol rated MGM targets: I don't know specifically what steel this is made out of, or how hard it is, but it is not rifle rated according to MGM. This is tougher stuff that the mild steel targets though, and will take a shotgun slug at close range without deforming. This stuff is more pricey and we won't use it if there is the possibility of it sustaining a rifle impact from a new shooter who shoots at the wrong target.
Budget AR500 steel targets: These are generally my personal targets which I will use at club matches. They are rifle rated-ish. Supposedly they are rated for anything under 3000fps, but they will show minor divots where steel is removed down to about 2800fps. These divots are generally about 1/16" deep. I have shot at them extensively with pistols at 10 yards and have never received a major piece of spall from them. I have also dented them with 165gr monolithic bullets out of a 30-06 at 100 yards. They have been shot enough to start to crack at the corners.
Premium MGM AR550 steel: These targets are excellent, but too expensive to employ in mass numbers. They have been shot with bullets easily going over 3000fps but have not had material removed. Their edges are also much stronger than the budget stuff, with no material being chipped away.
Hope it helps.
