I have, well, had, one of those Birchwood-Casey two plate spinners for handgun.
It was rated for up to .44 mag, and had about a 4" plate on top and about a 6" plate hung lower.
The plates were about 7/16" steel, and the rod for the frame was about 3/8" or so.
Anyway, I'd had it for many years, and punished it with countless thousands of .38's, .357's, 9's and some .44's.
The only damage it showed after all that was a bent leg from me trying to stake it into semi-frozen rocky earth. I stepped on the cross bar and one leg bent out.
Well, for some reason I shot it with my AR last fall.
I expected a little cratering, but I did not expect nearly-full penetration.
Like I said, the plate was 7/16", and the crater in it was also 7/16" deep, but had not poked through.
The back was dimpled out by about 3/32".
The diameter of the crater was about 3/8".
I was pretty impressed, although a little irritated I wrecked the spinner.
So, I shot it again, this time I hit the frame and blew the rods apart at a weld, so the thing wouldn't stand up anymore.
The ammo was just some 55gr. ball, out of my Bushy Patrolman's Carbine at 50 yards.
I'll try to find it in the scrap pile and get some pix if I have a little time.
-K