The ML-77 Missile Launcher, M-622 Avalanche, and M-920 Cain are researched within the Normandy SR-2's tech lab upon acquisition of enough Heavy Weapon ammunition upgrades the others can be found during missions or are obtained from DLC.
Finding these power cells is the only way to regain heavy weapon ammunition after expending it, unlike the ammunition for the other standard weapons types which is refilled automatically between missions. Rather than the universal thermal clips utilized by smaller weapons, heavy weapons consume power cells that can be found on missions.
