A ship has two critical dimensions related to the capsizing moment.

(1) The Center of Gravity; Cg
(2) the Center of Bouyancy; Cb

Cb is the center of gravity of the volume of water displaced by the ship.
A ship is designed so that the Cb is below the Cg on the same vertical line when the ship is upright. It is then stable. As a ship rolls, the Cb moves outward from the vertical line and rises toward the Cg. Once the Cb rises ABOVE the Cg the ship will not recover from the roll.

IRT to deck guns simply sitting in recesses in the deck, I don't know for sure but it sounds unlikely. The recoil forces and rotational forces of big guns would require them to be firmly mounted. If not so mounted normal movements, however small, would at best destroy accuracy and at worst damage the ship structure.

Warships are simply heavily ballasted to keep that Cb low.