Having a weapon attached to your arm that restricts the use of your hand is a common design trope in sci-fi-themed video games, such as Mega Man or Metroid. They even appear in the Marvel movies sometimes. But could such a cannon ever be more practical than just carrying a gun? In other words, what assumptions would you have to make about the world, the wielder, or the use case in order for an arm cannon to be the best choice of weapon?
For the purpose of this question, assume that:
- By "arm cannon", I mean a weapon that encloses or replaces a humanoid forearm and fires deadly projectiles.
- The arm cannon should be reliable enough to graduate from an R&D lab. It doesn't need to be cheap or mass-produced, but it needs to be useful.
- The user should still be fairly agile. If mobility was not a concern, they could just mount a turret.
- It can do other things (like serve as a replacement hand) if you want it to, but it's not a requirement.
- The extent that an arm cannon inhibits or requires normal use of the hand is unspecified. Perhaps the arm cannon relies on deft hand motions to operate, or it might even replace the arm entirely.
- The means by which an arm cannon is equipped or unequipped (if at all) is unspecified. Maybe it slips on like a glove at will, or maybe it's part of a larger exoskeleton that takes an hour to put on. Don't forget about mobility.
- The nature of the user is unspecified. Maybe an arm cannon wouldn't be practical for humans, but it'd be fine for humanoid robots that normally use hands, or some humanoid alien with more convenient biology.