The ACR "rifles" used smoothbore barrels to fire single or multiple flechette per round, rather than bullets, to provide long range, flat trajectory, and armor piercing abilities.
The top of the globe stands above the surface of the ocean, and running through the centre is a tower to add strength, and to provide space for homes, offices and even an amusement park for five thousand underwater residents.
A cylinder on the other hand does take advantage of some of the arch-like characteristics of a bathysphere, but aren't as effective in spreading out the pressure exerted on them as a completely rounded sphere.