I think you mean 300m from the target, not when the depth of your u-boat is 300m?
Because IIRC the game will not let you fire a torpedo at that depth, even if you got there and survived. I don't think it will let you open the tubes once you get too far below periscope depth.
At any rate, assuming we're talking about distance from target, torpedoes aren't even supposed to arm themselves until they've run at least 300m, so why they would be detonating as soon as they leave the tube is beyond me. I don't know if it's possible to mod the distance required for eels to arm themselves for detonation but if that's been tinkered with it might be the culprit (just a guess).
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