I have made several, for different purposes.
I usually use AR400 or AR500, which is armour plate. It handles soft lead bullets of any caliber. I have no exposed welds, the only welding is for a bracket on the back from which the target hangs. It has a nice ring to it when it is hit. Occasionally a weld will break, but that is easy to fix. I have used both 3/8 and 1/2 inch plate. The only difference I have noticed is that they sound different when they ring, and the 1/2 is heavy to haul around.
I would never use a jacketed bullet with these because the jacket will ricochet all over the place, even back towards the shooter! Since most of my firing is relatively close-range that's important to note!!!!
Don't ever fire 5.56 at these! I tried it once at 50 yards, figuring I would be far enough away to avoid any fly-back - which was true. But I destroyed my target. 5.56 does penetrate 1/2 armour plate. It doesn't go through. But it SERIOUSLY pitted the surface to the point where it becomes dangerous to fire at again -no longer being a clean flat surface. It took me a while to dig out the debris and reweld the pits, and grind it flat again - which leaves much softer metal in place of the 400 hard steel that used to be there! I weld the back bracket above the center back of the plate so that it hangs canted forward. That way all the debris is deflected down towards the ground.
On the other hand: our IPSC club uses a Texas Star target with armour steel plates and three different caliber rounds, none of which seriously damage the plates - but the debris flies out in every direction because the plates are not canted to deflect downwards.