It was just an idea from halo 3 that has not been seen in reach (Yet!)
This cannon can take two incarnations from its firing mechanism. The first idea is one that takes literally only a tap to activate. This tap against a custom powerup sends a fusion coil flying into another explosive block holding back whatever was in the barrel before firing. From there, Man-cannons will then increase the push against said object until it reaches lethal velocity. The other idea just uses a grenade to set off the charge. The trigger chamber can be inside or offset to the barrel.