Calico M950

One of the weapons Emiya Kiritsugu prepared for the Holy Grail War. You didn't really think he'd enter something like a survival game with just a sniper rifle and the Contender, did you?
The distinguishing feature of the Calico is its special top-mounted helical magazine that can hold 50 rounds, but no matter how much I think about it the extra weight and change in the center of gravity while firing means it would be better to use a normal submachine gun, even if you have to reload more often. But oh, well, Kiritsugu is a magical mystery gunman, so I guess it's fine.
Of course, in the final battle when he used it to pin down Kirei, he was able to buy himself five seconds where a normal submachine gun only would have gotten three. That extra two second advantage is a testament to the superiority of the helical magazine! …Okay, so it's not that much of a difference.
By the way, my intention was to make Kiritsugu as much of a heretical gunman as he is a heretical magus, so I purposefully chose his equipment based on coolness factor rather than practicality. Please, please, please don't think that the weapons that show up in Fate/Zero are the standard for actual professional soldiers.

Fate/Zero material: Fate/Zero Encylopedia