Get them quick and then be prepared to register them under NFA. Street sweepers were all the rage until the ATF reclassified them years ago.
This video is over 10 years old. And the fact that you do not see any for sale tells me he has not gotten the kinks out yet either legal or physical. Shotgun ammo does not work well in belts and those belts do not look like they lock the rounds down.
still want one...