In the 18th century, relatively low-cost and reliable rifles became available, and this fundamentally changed the balance between established governments and small armed groups. This changed contributed to the American Revolution, the French Revolution, and the downfall of the Mughal Empire (where small regional rebellions suddenly became more viable and central control began to break down).
This period eventually ended in the 19th century, when is debatable, but once you had a gatling gun, a professional heavily armed force could mow down lightly armed forces without large casualties.
Chances are this development will be the final word (and certainly it has not had a huge impact yet, so maybe my speculation is premature), eventually there will become some expensive but exceptionally effective weapon only large governments can afford or supply, but 3-D printers making guns could have serious effects on the course of politics and warfare within the next few decades.
As the old curse goes, may you live in interesting times...