Naval shells have much lower surface to mass ratios. In the limit that that ratio goes to zero the fluid forces disappear and you're left with simple parabolic motion + rotation of the Earth.

The fluid flow is the hard one!

You’re grossly oversimplifying. Naval firing calculations include factors like the temperature of both the air and the barrel and minor variations in powder quality - and then of course both the thing you’re trying to hit AND the gun itself are both moving in 3 dimensions.

Even WWII mechanical fire computers took 30-40 variables as input

But those factors are physically present in small bullets too, except they're ignored because other forces dominate.

Looks like +/- 2 inch accuracy which is well beyond naval guns at 20 miles.

Custom guided artillery shells could probably hit that kind of accuracy at significantly longer ranges, but pure ballistic weapons simply aren’t designed for extreme accuracy at range.

In theory it may not be possible for naval guns to hit targets 20 miles away. In practice it has happened more than once [1] [2].

[1] https://en.wikipedia.org/wiki/Battle_of_Calabria [2] https://en.wikipedia.org/wiki/HMS_Glorious#Sinking

Ships are a lot larger than a 2 inch radius circle. Even +/- 30 feet is more than good enough for a long range naval shelling.

Naval guns can have bullets 18" in diameter and weigh 3,848 lbs though.

Source: used to work at NSWCDD