It's a Junk-Box Special. The foregrip and handgrip are off a Daisy BB gun, the feed tube might have once been Nelson, and the bolt looks Breech-drop Nelson, either converted to dual-arm or out of a different gun.
It's 50 cal? That's odd... Sure it's not .62? (Which was a more popular caliber than fifty back in the day.)
The back half of the lower tube intrigues me. If the CO2 enters from the cartridge as normal, there's a long space between it and the power tube. Is it possible that rear screwdriver-slotted cap is a pressure regulator?
Post some good clear photos. The YouTube vid doesn't show much detail and you were moving it around too much.
Be careful dismantling it. Keep track of the parts and in what order they came out- especially if there IS
some kind of mechanism in that back half.