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One thing which virtually every spacecraft has in common is that is is made of a metal or alloy. This metal or alloy is typically affected by magnetic forces, which are often used to increase structural integrity.
To take advantage of this, the Magnetic Guidance Torpedoes have been developed. These function by using strong electro-magnets in the torpedo which are attracted to the hull of a ship, making them much easier to hit with (less compensation required for evasive maneuvers, very nice on fighters).
Unfortunately, because of the spinning forces created by the magnetic guidance, putting magnetic guidance on quantum torpedoes reduces their effectiveness to a level equivalent to that of photon torpedoes (because their "barrel" isn't correctly directed); photon torpedoes suffer no such disadvantage. In addition, the pull of the magnetic forces in the ship which fires these torpedoes slightly reduces their velocity.
~Jeremy Lennert, Simon Dark of the 7th Fleet