I have used AR500 - 600 steel to great effect, Sledge is right, do not weld, unless you design a target that has a long extension to make the attachment removed from the impact area. AR steel is hard to drill but can be done with Carbide. Simple rebar can be used for support of the target, a really simple swinger can be made with an angle iron hanger on the back, then it can be hung as a swinger, most metal providers will cut it for you for a minimal added cost.
If you have a shooting buddy or 3, the MGM flash target is hard to beat, last a lifetime, and Mike is a great guy, at 427.00 not cheap, but if split up 2 - 3 ways, it works so well and will last and last.
Just a few proviso's, if you use inferior steel, and get pock marks, those little craters, will send a bullet back almost as fast as it was sent, especially .22rf, I have been hit multiple times by this approach, I think that was T1 steel, over and inch thick, but thickness does not count, the impact surface, will get cratered.
Good luck sir.