The largest autoloading naval gun in service

What is the largest-bore autoloading naval gun fitted to an active-duty ship?

The US Navy's 8-inch cruiser guns were the largest autoloading naval guns in service.

The 8-inch automatic guns carried by American cruisers. They represent the upper end of the autoloading naval artillery actually discussed as being in service. That benchmark matters when evaluating modern proposals for heavy automated guns. It separates demonstrated cruiser-scale machinery from more speculative battleship-calibre systems.