I am shooting about a dozen steel plate targets from 4" swingers up to a 24" x 33" with everything from a .223 and handgun rounds to .338 Lapua.
The key to target durability and safety is to NOT allow the use of ANY sort of steel jacket or steel core bullet - all bullets must be either pure lead, regular cupro-nickel jacketed bullets or all copper slugs. Barnes bullets and Nosler Ballistic tips fall into the "OK" catagory.
I allowed a guy with an 8mm to shoot his steel jacketed milsurp at a few of the plates at relatively, and while the bullets did frag safely, they also chipped small craters into the surface.
I use a pair of cheap sawhorse brackets to make a sawhorse from 2 x 4s. The crossbar length only has to allow you to spot misses on either side of the plate - maybe an extra foot or so on each side. The leg lengths are enough to get the plate up out of the grass. On a hill, I use longer legs in front, so they'll stand level. Flat ground, all legs can be the same.
I use screw-hooks and chains to suspend the targets and they swing freely. I can e-mail pics of my setups to anyone who wants to see them.
First, you need VERY hard steel. Hot-rolled, cold-rolled and any other form of "mild" steel is a waste of time and money, and a safety hazard.
I used 3/8" AR500 for my own targets. This is harder than T-1 and is the customary material used by the commercial target makers. If a lot of heavier rifle shooting is anticipated, like you shoot a .338 Lapua exclusively, then 1/2" AR500 would be about minimum. With the 3/8" AR500, even at 1,000 yards, we got enough of a temporary dent to flake paint off the back side of the target and leave a *very* small permanent dent. I hate to use anything heavier than necessary. The problem is one of compromise. First, steel is sold per pound, and 1/2" is 25% more costly than 3/8". It is also 25% heavier, and my 24" x 33" target weighs 85 pounds. That is a lot to carry 20 yards up the berm and toss in and out of the pickup truck.
The ONLY bullets I've ever found intact have been pure lead pistol bullets, fired from a .455 Webley at about 50 yards - they were laying at the bottom of the target, totally flat and looked just like nickels. The next biggest pieces I've recovered were a few intact jackets that had expanded to be completely flat, and they looked like star-shaped flowers. They too were laying at the bottom of the plate. I found a few bullets bases, from flat based .30 bullets, that stuck into into the inside surfaces of the sawhorse legs like 1/4" frisbees. Everything else, including the 250 & 300 grain Lapua bullets, goes to dust. You can see the puff when the plate is struck.
Regarding deflecting the bullets off your frames with angle, your .223 will cut right through cold-rolled steel angle, but everything else should roll off it.
I agree with hoghunting, stay back at least 25 yards. If you use good, hard steel plate and non-steel bullets, you should be fine. Weekend before last, we shot (actually machine-gunned) mine with 7.62x39s, 8mm, .223s & .308's at 45-50 yards with nary a return frag, even from the steel jackets 8mms.
