Hi Don
The difference between a coastal gun and "any other land based gun" is this, the coastal gun has a big fat naval type rangefinder and naval type gunnery control centre, a ships "bridge/conning tower" on land.
This allows them to successfully engage ships far out to sea which a normal land based gun cannot.
So treating coastal guns(`150mm and above) and land based guns the same doesnt take this into account.
It would appear for Germany at least that pretty much any battery of guns over 150mm in a coastal defence role had a quality rangefinder and probably a control centre.
See,
http://www.normandiememoire.com/NM60...1_p2_13_gb.htm
http://www.normandiememoire.com/NM60...1_p2_03_gb.htm
http://www.normandiememoire.com/NM60...1_p2_01_gb.htm
http://www.normandiememoire.com/NM60...1_p2_06_gb.htm
http://www.normandiememoire.com/NM60...1_p2_05_gb.htm
http://www.normandiememoire.com/NM60...1_p2_04_gb.htm
http://www.normandiememoire.com/NM60...1_p2_14_gb.htm
http://www.normandiememoire.com/NM60...1_p2_22_gb.htm
and it is of course ludicrous to suggest that the italian 12 inch gun is mounted as coast defence without similar accessory equipment.
From
http://www.normandiememoire.com/NM60...sto1_p1_gb.htm
"more than forty guns of a calibre ranging from 105 to 240 mm(~9inch)"along a "30-kilometre stretch of sea front" resulted in this, "The Allied fleet, however, which had started bombarding Cherbourg during the attack, was kept at bay by the heavy coastal batteries."
Exactly the opposite is modeled in your game as shown by the comparison of the insect gunboat (accurate and AP round) to the Italian 12 inch coastal gun (inaccurate and AP round removed).
Is the reason the coastal guns have their AP removed because the indirect fire naval artillery has none? because if so it certainly gives the enemies on-board flotilla and landing craft an easy ride in The unrealistic combination of the coastal guns low accurracy and no AP means even if you hit anything you cant sink it.
As a rough guide assuming the insect gunboat has a rangefinder and that its accurracy of 30 isnt another typo then 30 could be considered it as a guide to what the coastal guns accuracy should be. As a shore based gun isnt moving, pitching, yawing or rolling the shore guns accuracy should be something better.
Best Regards Chuck.