Try to buy one of the most expensive and modern torpedo's and stack it in tube 5 (back tube), then start a game go to sea, then go to the torpedo loading screen, you will notice that tube 5 contains a grey torpedo and all the other contains the modern torpedos you bought.
This happens every time, I dont know why tube 5 wont track the torpedo you buy.
Btw. I am aware of the save-reload CO2 problem, but I just mentioned it because its still annoying.
