Yes, deck guns in RFB and TMO are nerfed to the point of real life. It routinely took over 100 shells fired over a period of 30 minutes or so to sink a ship. Some ships, like empty tankers were almost impossible to sink, even with torpedoes.
You also handicapped yourself by your shooting style. This tip works with torpedoes too, by the way. SH4 does not recognize additional damage in the same spot by subsequent hits. A dozen torpedoes hitting at the same spot won't do much more damage than one. Same with shells. In effect, you flushed 81 shots down the toilet.
The way to sink targets in SH4 is to distribute the hits along the water line. Then the ship will leak along its entire length and sink. With torpedoes, I almost always shoot two. The first is aimed at 1/4 of the length back from the bow, the second 1/4 of the way forward of the stern. This will play taps for just about any ship you want to hit.
Take advantage of the damage model. Don't shoot the same place twice. Good hunting!
If none of this works you might get rid of that geck gun and get a deck gun!:rotfl: