Have you ever heard of bending an Upper receiver?

[Rust]

Some will, many won't. Use cutting board as vice insulators, and do it sideways, so the rail is against one flat, and the bottom of the upper is against the other.

I know you've got a lot of experience working with AR's LRRPF52, but I've personally sprung/cracked a receiver with almost the exact same set up. I think it allows the potential for to much torque to be transferred into the thin wall below the ejection port. I've since moved to a clamshell type which supports the solid walls just behind the barrel nut threads.