At Merl's suggestion, I tried using a railroad tie plate for a target.
Here's the results.

I put two pieces of truck spring to keep the fragile chains farther from the target.

I shot off one of the washers keeping the 1/2" grade 8 bolts from slipping through the big spike holes.

Here's what it looked like after two AR's hitting it for several hours.
I think it will hold up for a while.

Here's a piece of truck leaf spring that we used closer in for 9mm and .40 S&W.
It's not even dimpling!

I finally tried drilling the leaf spring material. With some oil and care not to overspeed the drill, I got thru it with a 1/4" bit after a 1/8" pilot hole.
When I tried to go bigger, the material fought back hard and I broke a few bits proving that it's really hard stuff.
I used a hand drill, it would have been easier to control speed & feed with a drillpress - probably would have worked.
I ended up taking it to a local fab shop & they had a chop saw and a hydraulic punch that did the job.
Funny thing - the punch had a sign on it saying mild steel only. :mrgreen:
It really made a hell of a pop when the 9/16 punch went through.