Aetreus

It might be that the "shell weight" is the value used for tonnage needed for ammunition, in which case it would be including the propellant charge and extra tonnage for storage. Wouldn't be unreasonable then, 18" propellant charge was 403 kg so 2100kg total, plus some extra overhead would get you to 2400kg no problems. Also the USN did contemplate 40% overweight shells(so did Italy, weirdly), and those would be extremely heavy - the 16" 40% overweight tested was 3150lbs/1430kg.

Big guns(and each caliber increase within this) are definitely way too effective relative to small guns. Basic accuracy off of maximum range all the way through is definitely a big part of it. In general the way that guns effect accuracy is either via their precision- which was pretty much constant across all categories of guns and mostly dealt with their technology. Older guns were less stiff, had less consistent powder, had more problems with shell interference and blast effects, etc. This also applied to guns that were particularly powerful, though it could be mitigated. The quest for more power was frequently limited by the ability of more powerful guns to maintain consistent accuracy. The other main thing was time of flight, and therefore the ability of a ship to not be where predicted, magnifying errors in the firing solution. This is related to maximum range but primarily when ranges were relatively far. So for instance the USN 8" gun has a time of flight of 14.7 seconds at 10k yards, versus the 16"/50 having 13.2 seconds, a difference of only 11%. But at 30k yards, near the 8" guns maximum range, the difference is 77.8 seconds versus 50.3 seconds. A 54% difference! Basically, at very short ranges small and large guns should have almost identical accuracies if they're using the same fire control. As range increases accuracy should drop similarly for all guns, with a "cliff" near the maximum range where the accuracy drops off much more quickly. As tech improves, smaller guns gain range faster than larger guns(basically, big guns have longer range because they experience less drag, so overall drag reduction impacts small guns more).