This is the Visby main weapon, the Bofors 57 mm Mk3 in its folded position. This part is hollowed out for the lightest it can be so there should not be any tipping factor.
Wiki:
The 57 mm Mk 3 in a stealth cupola aboard a Visby-class corvette, the stealth version of the Mk 110 57 mm is similar in appearance