When your crush touches your arm

If that's the size of your gun I'd hate to see how big your cannon is

aren't cannons just big-ass guns?

eh? different stuff goes into how it works and from a legal distinction a fire arm has to be able to be used manually in your hands with your hand physically activating the firing mechanism. Modern cannons are a lot more similar to guns than the stereotypical musket era cannon is but they still work differently. The only thing that really makes them super same-y is the combustion propulsion of the projectile.