This represented a breathtaking departure from time-honored gun-making technology, in which a gunsmith would make one gun at a time, part by part, with no two guns exactly the same size. When a rifle malfunctioned and needed a new part, that piece had to be custom-fitted by a gunsmith.

By contrast, parts made by the Robbins and Lawrence Armory were interchangeable, because they were precisely the same size. They'd been made not by hand, but by machines. Those machines had been assembled, refined, and in some cases, invented by Vermont craftsmen who not only set a standard other northern industrialists sought to match, but helped prepare the way for mass production of consumer goods that dominated manufacturing through much of 19th and 20th centuries.

The native ingenuity that helped transform American manufacturing survives in a remarkable collection of factory artifacts in the old armory itself, now known as the American Precision Museum. Gun-stock lathe, drill press, and assorted milling machines are all on display, along with a rifling machine that cut spiral grooves inside gun barrels to improve accuracy. Equipment like this was once state-of-the-art and key to the production of the Union weaponry that won the Civil War.

But economic innovation evolves in unexpected ways, and the American manufacturing revolution has now come full circle, from mass production to the one-of-a-kind, customized output of a 3-D printer. What really completes the circle, from the armory days to now, is that a 3-D printer can be used — and has been used in Texas and Japan, controversially — to make a gun.

In a nod to modernity, the museum has a 3-D printer on display on the shop floor among the antique-but-intricate contraptions that require antique skills to operate. It looks out of place, like a nose ring in a Victorian mannequin.

Read more