Except it was out of character to have McCoy be okay with working on a weapon. In "The Empath," he said, "I will not take a life, not even to save my own." He's a doctor dedicated to doing no harm.
Also, I just don't buy that excuse. Yes, surgeons have precise hand-eye coordination, but I'm sure plenty of engineers and weapons technicians do too. And I just don't buy that the skill set tailored for doing delicate work with soft, squishy body tissues is translatable to doing delicate work with wires and circuit boards. Hell, earlier in the same movie, McCoy had been clueless about how to treat a wounded Klingon, and at least that's the same general type of problem as treating a wounded human, even if some of the specifics are different. Yet somehow he's perfectly adept at handling the radically different type of problem that modifying a torpedo would entail? (And I have a huge problem with his cluelessness about Klingon anatomy, too. Thirty-plus years in Starfleet and he's never had occasion to study a Klingon patient or cadaver?)