That has been a common trope in gangster movies for ages. A violin case can easially hide a gun (depending on the gun of course, but you get to select this).

That has been a common trope in gangster movies for ages. A violin case can easially hide a gun (depending on the gun of course, but you get to select this). You can find craftsmen in your gang who can make a violin case that also has a gun so it can even be opened an inspected by someone who doesn't know how to find the secret gun compartment, and it isn't hard to teach someone to play some simple songs on a violin thus giving a reason to carry a violin (you don't need to be good, just be able to show you have another lesson next week). I don't know if this works as well in the real world as movies, but it is plausible.