50 cal blackpowder?? maybe, if you are talking about .50 bmg, even if using 500 Brinell steel, the toughest steel for professional military targets, the projectiles will go right through like butter, you may get it to spin, but they will get many holes, and then loose weight and then will not spin due to lack of weight, impractical for .50cal
Oh, still a hot round, 1/2 inch 500 Brinell is the only steel that will really work, but we are in the right area for it, ship yards should have some, find some old propellers / drive train, that is where it originates from, bullets will not ding it much, some of the scrap yards such as SOuthern Scrap in NOLA may have some as well
Call the local steel shops. They sell it by the puond and will cut out what you need for little or nothing. Scrap prices are going to cost almost as much and then you have to get it cut to your needs.
Shipyards/scrapyards ain't gonna have what you need. If you do get a deal on some 1/2" or thicker steel just fabricate/replace as necessary. Good excuse to buy a welding machine and oxy/act rig.
We've got some 4" x 120" x 240" grade 70 plates out in the shop that you could buy and we could cut whatever shape you wanted with the plasma burner. I'm not sure how you'd move them though, as that stuff weighs 163 lbs per sq ft, but I'd bet a $100 they would stop a S&W 500!
We do sell everything from 10ga steel sheets to 6" steel plate, channel, angle iron, wide flange beams, I-beams, expanded metal, rebar and welded wire fabric. I don't know if I can cut you any kind of deal because I work only in the rebar department, but I can put you in touch with someone in our structural sales department.
Once you figure out what you're looking for, feel free to PM me if you want some contact info.
We can olny shoot 357 mag (maximun) at the steel at IPSO and it must be at 15 yards or greater due to this problem. I've been hit with 38 special from 15 and 25 feet and 45 ACP from 4 yards (hince the 15 yard rule) - no significant damage but I can imagine what a 50 BMG would feel like. Actually I can't imagine, but I know it would hurt.
I was thinking about rolling the top around a peice of rebar and making a spinning target out of it
Not sure if they would even let me set it up over there