Stock SH4 has the reload time set to 4 seconds.
Note that the navweaps site lists the ROF for that weapon as 15-20 rpm. Poor
Bowfin didn't read the manual, or they would have fired faster I guess
So 66-100% lower ROF than spec, probably due to the poor platform a submarine is for a gun, ammo issues, and everythign being soaking wet all the time.
I'm actually perfectly happy with ROFs being in the ballpark of what that
Bowfin site claims as long as the guns aren't gyro-stabilized death rays. Firing for effect, lying to in a calm sea, that would be fine as long as a boat making flank into a stiff sea can't do the same. Kv29's mod looks like it will absolutely mitigate this problem and allow a more accurate ROF to be in game while preserving historical outcomes.
That said, if the real ROF was 10 (assuming plenty of ready ammo), I'd tend to drop it by some small % to cover problems still not able to be modded in. So maybe 8-9 rpm for a 10rpm gun (jams/misfire clearing, etc).
Not that in my testing of kv29's mod, I was lucky to shoot at ~8 rpm at close range with the reload time set to 4 seconds just because of the roll and pitch of the boat.