Kali, not sure what you mean? Comparing what hole? The hole that the barrel fits through? The barrel is .75 under the front of the block, but something like 1.25" under the back end of the block. I forgot to measure the barrel, and I didn't remove the block, but there is a straight step down between the two, which butts up against the inside of the collar. If memory serves, there's a milled counterbore for the hex screw to hold the collar on, and there is a similar one on the top side where the gas tube group is held to the barrel. There's a small steel gas seal between the barrel and the gas tube. And the gas tube has a very small block pinned to the end, in that half round shape you can see in the front view. You should look up the old 231 thread for specifics on those. Someone measured and had manufactured some of those gas seals, that's where I got mine.
As for Bradleys, they're still in active service in the Military, so you won't find them in Civilian hands, or at Civilian museums until they get retired. You may be able to find them at military run museums like Ft Knox. . . but I'm not sure. Even then, most of the ball mounts were covered over, so any in museums may not have the mounts. Still, I'm with you, I'd LOVE to see my clone hanging in the back ramp of a Bradley. Or get to see what it was like to fire out the side while looking through a periscope.
VAAR, it's definitely steel. Those threads would sheer off in the ball mount if it were aluminum. Still, for our civvy use. . . I don't think there's any reason you COULDN'T make it in aluminum.