Just load up swdw/LukeFF/kv29's Real Deck Gun and see what happens when you don't have a gyro-stabilized deck gun. You have no clue how bad the real comparison is from submarine to Japanese warship or even armed merchant. Comparatively, you have a laser guided, gyro-stabilized machine gun. Enjoy.
Edit: It gets worse, not only is your gun laser guided and gyro stabilized, but if you're not using TM/New Real Deck Gun you're also shooting photon torpedoes that will sink a target in at most a couple dozen shots. My deck gun takes at least 100 hits to sink just about anything larger than a sampan. A normal sinking takes 20 minutes real time minimum. I don't usually attack uninjured opponents for that reason.
Edit Edit: check out this thread for a full explanation of the deck gun and its use in SH4. This is from the last version of Real Fleet Boat that I ran.