I designed my swing target in 1985 and had it built by a friend. It is made from 1/2" plate steel, and the legs and support are made from a 9' piece of 5/8" stainless rod. I do not remember the cost of the material, but most of it was scrap or very cheap. Ever so often I spray paint it black so that I can see the bullet strikes. I did paint it white once, but I prefer the black. YMMV. I had a friend come over to try out his new .45 ACP last weekend, and as you can see from the splatter, all parts of the target will be hit including the rod.
As you can see from the picture, the top of one of my targets has broken off. It had stayed intact for 25 years until 3 or 4 weeks ago. It has had untold thousands of rounds impact it in that length of time. I have used every caliber of pistol ammunition on it including very hot .44 mag. rounds. I would not recommend .44 mag. or .357 mag. on the target at close range. I used a .44 mag. at a distance of about 60', and it left some very distinct craters with bulges on the back side. The only rifle that I have used on this target is a .22 caliber. I do not believe that a larger caliber rifle will penetrate the 1/2" steel targets at 100 yards, but I have never done it.
The cross piece of 5/8" rod is 3' across. I had a coupling cut in half as my stops and welded with about 3/16" gap between the stop and the rotating coupling on each side. The center couplings have been hit many many times, and it has not affected the targets ability to swing.
The overall length of the target is 19". The bottom target is 6" in diameter with a 3" tail welded to the coupling. The top target is 3" in diameter with a 7" tail which is welded 180 degrees from the bottom target. I made the tails about 2" in width.
As you can see from the picture, there is about 10" center to center.
I then bent the 5/8" rod 90 degrees at almost 3" on either side. The only down side to this type of swing target is the weight which is about 30 pounds, and had I used thicker plate, I would have been able to shoot hot magnums into it at close range. I no longer shoot the hot magnums. IMHO it is a waste of powder, will wear out your firearm faster, and a lighter load will have the same effect on soft tissue without collateral damage.