The White Round Time Machine is a machine made by Shoichi to intercept the original place-switching of the Ten Year Bazooka. It was revealed to have contained the future Tsuna and his Guardians as well as some of their friends inside as reduced molecules. It also held the Vongola Box Weapons until Shoichi took them out and distributed them among the past Tsuna and his Guardians.
This machine is actually a time machine and can send them back to the past. The machine also has the capability to set the amount of time that would pass in the past during the time-travelling person's time in the future.