Not sure if you're playing stock or what, if any, MODs, but...
Another typical instance when an impact pistol fails to detonate is when the torpedo strikes the turn of the bilge. If you're hitting the target anywhere less than 2m above the keel, you're likely to experience that type of dud. Be careful to set the depth with impact detonators at least 2+ meters above the keel depth. You should also account for sea state and the up and down movement of the target due to wave action. I've always found the best bet is usually to set them for 3-4m above the keel depth, 4-5m in rough seas...you will have problems even then when wind speed is 13-15m/sec.
With magnetic pistols, you may also experience failures when the sea state is high, due to the movement caused by wave action. If you're setting the depth at 1-2m below the keel, that movement can cause the ship to rise up 3-4m above the torpedo depth setting or fall 3-4m below the torpedo depth setting. In the former case, the detonator fails because it is too far below the ship's bottom. In the latter, I've observed it fail because it hits the hull on the turn of the bilge...neither mag pistol nor impact pistol will function in that case.
There is also one other factor to be careful of...torpedo arming distance. If you're firing at 400m, the torpedo track to the target point of impact might be 350m or less. IIRC the arming distance is 350m...maybe 300? Anyway, short-range shots like that might be too close for the detonator to arm.
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