Why Soviet cruisers used 180 mm guns

Why did the Soviet Navy choose 180 mm guns for its cruisers?

A modern 180 mm railway-gun design was already available, easy to adapt, and offered good ballistic performance.

The Soviet Union continued developing turreted railway guns during the period when it was not building large warships. Work in the 1920s and 1930s therefore left it with a modern 180 mm weapon when cruiser construction resumed. Adapting that gun for a cruiser mounting was comparatively easy. Its long barrel also offered good ballistic performance.