If I'm reading this correctly, it allows you to deal 1 damage every turn you're in hero form, and also converts all your leftover ammo to damage 1:1 every time you flip back to alter ego. Combined with Munitions Bunker, you can stockpile some pretty ridiculous amounts of damage, accessible whenever you'd like. I think it's probably War Machine's biggest strength, the ability to on-demand nuke the final phase of a fight.
Sure, other cards like the Missile Launcher convert ammo to damage more efficiently, but for resources paid and burst damage (and pure flexibility), Shoulder Cannon is a monster.
just don't run into an enemy with Retaliate.