We have several ship breaking yards where they cut up old WWII vessels and I was given a piece of 3" thick armor plate which was about 18" square. It was supposedly (torch) cut from a gun turret. I welded a piece of 3" angle iron on the back of it (since I'm a dauber, not a welder, I was pretty proud of my welds which were some of my best looking beads.) but when I pecked it w/chipping hammer, the plate fell and I barely jerked my foot out of the way in time to avoid a mashed toe!
Turned out to be cast iron, so I drilled and tapped two 1/2" holes and bolted the angle iron leg in place. I was surprised at how long it lasted @ the 200 yard berm at our club range. It got a lot of use and the heavier calibers would dimple the face about 1/8" deep. I would think that the manhole cover would hold up fairly well @ 800 yards but would avoid the magnums.
Regards,
hm