Thank you guys, we really appreciate your input
I should probably start by mentioning that it is possible to BOL-launch torpedoes if you want to be creative. You can then manually stear them to their target at max range - granted the weapons have wire guidance and you don't fire more torps than you have wire guidance channels. The 6nm range limitation is for "ordinary" torpedo shots.
We actually tried a more advanced method for determining max firing range based on torpedo speed and target speed & heading. This resulted in a lot of stupid shots while the AI was building a firing resolution. Like, if you have contact at 20nm but no speed info, do you shoot? No? But if the same contact is at 4nm? Etc etc.
The end result was that the targets fled and helicopters scrambled for no torp hits. A diesel sub in this situation is in pretty deep poo-poo since it doesn't have the speed to catch up with the SAG or fast convoy, and also lacks the speed and diving depth to get outa there before the helos start dipping their sonars.
This situation got even worse when the opening long-range shot was against one of the outer screen ASW escorts. The SAG/convoy would turn around and run, and there was no way an AI-controlled submarine would get into position for a shot on the HVUs.
As such, limiting the firing range from 'max theoretical' to 'max practical' the AI was much more successful. It would wait for the targets to get real close before shooting off torps and sinking ships in quick succession.
The AI still does the target/speed evaluation, but practical firing range is limited to 6nm/8nm (depending on what we do next) for the above reasons.