Guardians undercover(Katekyo Hitman Reborn F...
Gianini makes a new modified version of the Ten-Year bazooka, a new version that will allow the future selves stay a longer time than just 5 minutes. But something ends up going wrong when it's tested on Tsuna and his friends. Somehow they ended up in the past with the First Generation, and they've shrunk.
Might have a slight crossover, maybe. Not sure how many parts.