It's in the SH3\data\Cfg\Basic.cfg file. IIRC you start by scrolling down to the '[CONING_TYPEx]' section near the end of the file. Once there you can edit the availability dates for the various turms.
Next you have to go up (in the same file) to the '[SUBMARINE_AMMOx] entries for your sub type. This is where you specify the weapon loadout (deck, flak and torpedoes) for your boat in any given year.
Again, IIRC the MO1 entry controls deck gun type, while ZO1 sets the turm and the AO1 (and for multiple guns - AO2) configure the flak.
To figure out which is which, you need to Ctrl-F down to '[AMMO_TYPEx]' and read the idx identification number. In other words, if you wanted to configure ammo for an 88mm gun you would enter M0=1. The '1' being the corresponding idx identifier.
The next step is to make sure you're in the right year as the game assigns an id to each year starting with 1939 as 00, 1940 as 10, '41 as 20 and so on through to 1945.
So the actual entry (in the submarine ammo section) would read 'M01_00=1' for 1939.
Last but not least, to select flak, Ctrl-F down to '[A01]' and grab the idx number that corresponds to the type you're looking for. Just make sure you've also changed the availability date so you can get the desired type in the right year.
Hopefully I haven't left out any steps, but if so kindly pardon me in advance as it has been a while. Good luck and happy modding!
