Yeah, when you pile up reinforced steel plating to a thickness of several inches, and add all of the mechanics for the turret, it's going to get pretty heavy. After all, if the turret gets breached, you can pretty much kiss the rest of the ship goodbye. (Do you know how much explosive ordinance was stored under those things?) That seems low to me though, because the average main battle tank weighs in at around 70 tons, and their armor isn't NEARLY as thick as a battleship main turret.
And it all depends on whether you're going HT&L or not, but no matter how you cut it those turrets are just too large.