Getting the steel itself is pretty easy. You're looking for high wear type steel used in mining operations or like you'd find on snow plows. I have both AR400 and AR500 steel. They are extremely resilient against most common rifle and pistol rounds, and that's a good thing because depending on your sources, armor plate is either going to be really inexpensive, or extremely expensive. The best place to start looking is in your local classifieds under guns. You can also put out a WTB (want to buy) ad stating exactly what you want. I will go more into that in a bit. First off, let's get to building... because building shit is stuff that real men like to do.
(x1) Armored plate (AR400 or AR500) in the shape and size of your choice. This is a 12x12" AR400 steel plate that is 1/2" thick. It can take just about anything up to a 308 WIN. Just don't shoot a lot of 7mm Mag against it because it's not strong enough for that. If you want to shoot big bullets, upgrade to 1/2" AR500. Oh yes, and this metal is not easily drilled. Have your holes pre cut with a plasma cutter if you can. Offer to pay a few bucks extra. It's worth it.
(x5) 2x4's in the length of your choice. Since my steel is only 12x12", I bought 2 pieces of 96" 2x4's and had them all cut to 32" in length. That gave me 6 pieces. You only need 5. Keep the spare one. You'll see why later.
(x2) Metal Sawhorse brackets. I bought these at Home Depot. They are inexpensive and you won't cry if you shoot them to death.
(x2) Eyelet bolts. These are rated at 75 lbs per eyelet.
(x2) Cheap carabiners. These are rated at 150 lbs per carabiner.
(x2) Can of spray paint. One can has your primer. The other one is for the target area.
Drill bit, sized for the shank of your eyelet bolt.
Wrench, or some other tool that can get into the eyelet. You'll see why.
Safety Glasses. Even though this is a simple project, it's still technically wood working, and I can't seem to get my Jr high school shop teacher yelling "Stop horsing around and put your safety glasses back on" out of my head
Organize your work space. Put the carabiners through the holes in your steel plate, and sort of center up the wood you intend to use to hang the plate from. Make sure this wood is free of cracks, knots, or other defects. Align your caribiners as shown above and drill two holes that your eyelets will be screwed into. No points given for precision here. That wood is going to get the crap kicked out of it once you start shooting it, as you can see below:
Get your eyelets to the final depth and orientate them so that there is no binding with the carabiners, as shown above. Pick up the combination by the ends of the wood, and verify no binding as the plate swings back and forth a bit. If there is binding, get bigger carabiners, eyelets, or both.
Sadly, we are done playing with power tools and hand tools for the moment. Assemble the saw horse by inserting the legs into the slots provided on the hinge. Do this for both sides. Then stand one side up, and just like you see in the picture above, place the cross beam and clamp it down so it doesn't move. Note: Do not have the steel plate attached when you do this because it'd be a bitch and you could really hurt yourself if it falls apart while trying to assemble it. Anyway, prop the other legs up and clamp them into the crossbeam. Now spread the legs out. This will do two things: It will create a bigger footprint, and it will allow the hinge to clamp onto the crossbeam so it doesn't move when you shoot at it.
Attach the carabiners to the plate first. Then carefully attach them to their respective eyelets. Voila! You have a self-supporting target stand! The stand itself becomes stronger when the plate is attached. Like a suspension bridge, the weight bearing down on the crossbeam exerts downward force onto the hinges, sort of locking them in. What I like to do is kick the legs out at the bottom just a bit more so they plant onto th ground better. You can use sandbags to stabilize them if you want, but my 4 ft high stand (that I use for my 18" plate) doesn't need it.
Cost to build without plate: Around $12.
Paint your plate. I used a white primer. This will not only protect it from corrosion while in storage, but it presents a nice contrast against the scenery you'll be shooting in. I have found that white works extremely well in the desert, especially if the target is placed in an area with darker foliage.
Step 7 (Optional):
To make a simple bullseye, I repurposed a pizza box. For this tutorial, I'm using my 18x18" plate as an example.
To get a perfect circle, you need a compass of some kind. I found just the solution inside the box my Dremel Tool came in. Overkill? Yes. But this is so me.
Place the newly made stencil over your steel plate.
And paint with the color of your choice. In this case, United Nations blue (or the closest I could find) does the trick nicely.
The finished product! It doesn't have to be perfect. Believe me, after you take a few shots at this mother, your hard work won't look so awesome no more. Just eyeball it and run with it. Again, no points given for precision because you are going to take out a lot of pent up anger and frustration on this thing!
Now, there are general rules regarding safe shooting of steel. Generally, a rule of thumb is no closer than 25 yards. I've shot steel at point blank range before without any problems. Because these saw horse targets sway when you hit them, the fragments go straight down and up - not back at you. A 5.56mm round destroys itself on impact with this plate. Still, I would wear safety glasses and a medium shirt that has full sleeve coverage, as well as shooting gloves. Never shoot while wearing shorts. Wear good pants and medium to heavy boots. That's just a good rule in general.
Obviously, the consumables in these targets are the wood legs and crossbeam as well as the carabiners and the eyelets if you blast them enough. The hinges hold up pretty well if you don't shoot them. I have a bullet hole in one of mine from another person who came shooting with me. I make it a point not to shoot my target stands because even though they are cheap, they still cost money. Your initial investment with vary, depending on where you source your steel. But the hinges run about $6 bucks before tax. I paid about a buck for the eyelets and a buck apiece for the carabiners. If you go to Harbor Freight, you can find carabiners that'll work just fine for about 30 cents. The wood ran me about $5 because the studs I found were 96 inches long. Divided by 3, that's 32" per piece, and you need 5 pieces. 2x4's are easy to find and cheap. You can check classified ads for people unloading them from a demolition project or people looking to get rid of scrap wood. If you go to Home Depot, you can get them cut to your necessary size before ever leaving the store - and the boys in the orange aprons are happy to cut it for you. I do it this way just because it's way easier to put them all in my Jeep and haul them home instead of lashing them to the roof, getting out my saw, and cutting them myself. I save time. It takes the guys at Home Depot all of 2 minutes to saw them to your specs. And you don't have to clean it up.
Provided you take care of your target stands, you should expect multiple outings before changing the crossbeam. The legs will get beat up too, but not nearly as bad as the crossbeam. You should be able to get a lot of outings out of them before it's time to replace. If you live in a wet environment, it might not be a bad idea to put a coat of paint on the legs and crossbeam to protect against moisture. I live in the desert, so I just leave the wood as is, and store it in my garage.
Here's a short video of that small AR500 steel plate in action. It's fun to shoot steel!