Kiritsugu is a debatable example. His gun is only effective because of the Nasuverse's ridiculously complex magic system. Regular bullets would do next to nothing against most of that setting's threats.

Then again he's one of the most feared mortals (despite being kind of shit as a Magus) due to simply sniping his targets when their defenses are down or demolishing their entire hideout with copious explosives - he's the epitome of "boring but practical". The handgun and its bullshit anti-magic bullets only come out on the rare occasion where he confronts his targets face-to-face.