That is more like a mortar than a rifle. I'll keep my Pauza P50 BMG semi-auto carbine. Available ammo, flatter trajectory , much better penetration, less recoil, and no Class III destructive device issues. Ok, it's 3000 ft-lbs less muzzle energy but I'd bet it has more energy at over 500 yards. I don't know the BC of the 70 cal cast lead bullet, but I doubt it's as high as standard 50 BMG ball.
Come on, you know you want to shoot it once or twice.
Yup. "I have the biggest one." is still a valid claim no matter how impractical someone tries to make it sound.
If I built a 700 WTF it would not be "the biggest one" in my neighborhood. A nearby rancher has a 25mm Hotchkiss semi-auto rifle similar to this: http://themaginotline.info/25mm_1.php
You can't win the "mine is bigger than yours" game unless you have really deep pockets.