Alternate History, WWI:
The Pedersen device was designed to turn a 5 shot heavy caliber bolt action rifle into a 30 shot light pistol caliber semiauto. The idea being that once you get into the trench fast and furious fighting is more the norm.
The Pedersen device was a colossal failure, only spared from historical ignominy by the fact the war ended before it was deployed. But the idea is incredibly cool. Beyond that, it's an illustration of the contradiction of the traditional trench war. The armies were organized around full power, long range range rifles, but once you get into trenches, these rifles are nearly unusable and pistols and submachine-guns become more ideal.
