Steyr AUG

The assault rifle used by Hisau Maiya. It seems it's one of the better weapons prepared by Kiritsugu.
As the first successful example of a bullpup rifle, it must have seemed almost bizarre when it was first released. The fact that it consistently outperformed subsequent bullpup rifles, and is even used as the official weapon of many militaries to this day is, quite frankly, amazing.
One of its primary features is its easy-to-disassemble modular construction. However, thanks to the popularity of scenes like the one from Day of the Jackal where the assassin assembles their weapon on the spot, it's often mistaken for a sniper rifle in movies. Well, it does have a sort of “secret weapon” look to it, so I guess I understand.
…Ah, wait, now that I think about it didn't Maiya try to snipe at Kayneth's hotel from the neighboring building?

Fate/Zero material: Fate/Zero Encylopedia