Stars and Stripes in Peril by Harry Harrison

Chesapeake & Ohio railroad be built, coming right into the shipyard. It was already proving its worth, bringing iron plating right to the dockside.
    But this was no ordinary cargo of iron. The train consisted of a single passenger coach behind the engine, with a heavily laden flatcar behind that. A stubby man in a frockcoat, wearing a black stovepipe hat, climbed down from the coach as Ericsson came up.
    "Could you possibly be Mr. Ericsson?" he said, extending his hand. "My name is Parrott, William Parker Parrott."
    "The gunsmith! This is a great pleasure. I have designed guns myself so know of what I speak. And this is the 12-inch cannon that you wrote me about."
    "It is."
    "Beautiful," Ericsson said as they both stepped back to admire the bulk of the long, black gun. Beauty is in the eye of the beholder for this was a hulking black engine of destruction. "Your locking breech, this I must see at once."
    They both clambered up onto the flatcar, in their enthusiasm not noticing the soot that smeared onto them.
    "The gas seal," Parrott said, "that is the heart of a breechloading gun. I have examined closely the British Armstrong cannon, have even built one of them. Its breech is complex and when firing begins it soon becomes unusable. A sliding metal plate is secured in place by large locking screws. But the seal is incomplete. After a few rounds the heated metal expands and leaks hot gas and threatens the very safety of the crew should the breech explode—as has happened more than once. But I believe that I have now solved that problem."
    "You must tell me—show me!"
    "I shall. The principle is a simple one. Imagine, if you will, a heavy threaded breach, into which a threaded bolt can be screwed into place."
    "The gas seal would be complete. But it would be the devil's own job—and a slow one at that—to screw a long bolt in and out between each shot."
    "Of course. So let me show you..."
    Parrott went to the rear of the gun and reached up to strain at a long lever. He could barely reach it, nor was he able to pull it down. The taller Swede who, despite his advanced age, was immensely strong, reached past the small gunsmith and pulled the bar down with a mighty heave. The breech-block rotated—then swung aside on a large pinned hinge. Ericsson ran his fingers over the threads on block and barrel.
    "It is an interrupted screw," Parrott said proudly. "The theory is a simple one—but getting the machining right was very difficult. As you can see, after the breach and the breechblock have been threaded, channels are cut in each of them. The block then slides forward into place. And with a twist it locks. A perfect gas seal has been accomplished by the threading. After firing the process is reversed."
    "You are indeed a genius," Ericsson said, running his fingers over the thick iron screw threads. Possibly the only time in his life that he had praised another man.
    "If you will show me the ship on which it will be mounted..."
    "Difficult to do," Ericsson said, smiling as he tapped his head. "Most of it is inside here. But I can show you the drawings I have made. If you will step inside my office."
    Ericsson had not stinted himself with the government's money when he had designed a workplace for himself. He had labored for too many years in the past in drafty drawing rooms, sometimes only feebly lit by sooty lanterns. Now large windows—as well as a skylight—illuminated his handsome mahogany-framed drafting table. Shelves beside it contained models of the various ships he had designed, other inventions as well. The drawing of Virginia was spread across the table. He tapped it proudly with his finger.
    "There will be a turret here on the forelock, another aft. Each will mount two of your guns."
    Parrott listened intently as the Swedish engineer proudly pointed out the details that would be incorporated into his latest design. But his eyes kept wandering to a chunky metal device that stood on the floor. It had pipes sticking