It's funny, there are two approaches, steel or rubber. I haul concrete to get a paycheck. That means I'm around mud flaps and conveyor belts all the time. I use chain. Go figure.
I go by the theory that nothing will hold up. Up keep is constant. Chain can be found may places. I'm not opposed to picking up chunks of chain at the road side. I find all sorts of stuff, tools, stuffed animals, and stuff at the side of the road.
My main range has two 6x6 pressure treated posts sunk in the ground with concrete. I nail scrap 2x board across the front and back. In between I dump 'pit waste' that is the waste that all concrete plants generate. Every one of you live within a dozen miles of a few concrete plants, and every one of them pays to have someone haul the stuff off. You can get it for nothing if you have any skill at asking. Dump it between the boards and nothing goes thru it. I've not tried .50bmg's, but .30-06 and Casulls at 25 yards never come out the other side.
Across the top I have a roof to keep the thing dry, and I have two 2x4 arms that stick out towards the shooting bench area. I hang chain from that. You can pick up plate steel all over the place. Go to a recycling place. They will let you run off with some. Worst case it might cost you a buck or two. I'd bet if a person bought a low dollar cooler, loaded it with sodas and went to a steel recycle place, they could get anything they wanted.
Another construction waste item is the twist ties that they bundle rebar up with. It is a very soft malleable steel about a 1/4 inch in diameter. Works great for fixing chains or hanging steel. I keep a stout pair of channel locks, a hammer, and some nails down on the range. This lets me fix stuff up as I tear it apart. No money out of pocket.
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