By the time the Civil War ended, there was no doubt that the self-contained metallic cartridge was the wave of the future. With the successful, widespread use of the Spencer repeater as well as more limited employment of other arms such as the Henry and Joslyn, the message was clear: The percussion muzzleloader and breechloader were doomed.

Other countries were also in the process of switching over, and the United States didn’t want to be left behind. With hundreds of thousands of rifle-muskets on hand, the obvious money-saving solution was to try to find some way to convert these arms to handle the more modern ignition system.

While some excellent newly made arms were submitted for testing, the prudent thing to do was to adopt a method that would be able to make use of the resources at hand.

The Yankees were not alone in this, and nations such as Britain, with its Snider conversion, and France, which adopted some muzzleloading muskets and carbines to the Tabatier system and Chassepot bolt-actions to handle the 11mm Gras cartridge, were also going in the same direction.

Uncle Sam tried many different arrangements before settling on one devised by Springfield Armory’s master armorer, Erskine S. Allin. As well as being a slick setup, Allin was a government employee, which meant the war department would not have to pay royalties on the design.

Basically the system involved a breechblock that was attached to the top of the altered rifle-musket barrel. To open it, one half-cocked the hammer and pushed upward on a thumb latch to unlock the block. It would then be rotated forward to expose the chamber for the insertion of a round.

The block was then closed, the hammer put on full-clock, and the rifle aimed and fired. Upon reopening the block, a rather complicated rack-and-pinion extractor ejected the spent case. Fast, slick and easy.

As well as being relatively efficient, the conversion allowed the maximum use of original parts. Allin explained: “It is particularly adapted to the alteration of the Springfield rifle-musket, (or any other) as it can be done without changing the features of the musket or without throwing away any of its parts. All that is necessary is to cut away the barrel on the top at the breech and add the block and shell extractor, cut the recess in the breech-screw, and modify the hammer. All other parts remain the same.”

The Trapdoor's top-opening breechblock allowed the upgrade from black powder to smokeless while still retaining as many original parts as possible.

These were honeyed words to penurious ordnance officials. The system was adopted in 1865, chambering a .58 caliber copper rimfire cartridge that approximated the ballistics of the original muzzle-loader’s paper cartridge. Because of this, even the rear sight didn’t have to be changed.

Nicknamed the “needle gun” due to its long firing pin, some 5,000 of these Model 1865s were altered at Springfield before it was decided that some alterations in the mechanism and a reduction in caliber was warranted to further improve the gun’s performance.

The original muzzleloading barrels were sleeved with a liner of .50 caliber to handle a new centerfire cartridge with a 450-grain lead bullet backed by 70 grains of black powder. This loading upped the velocity by 100 fps (1,260 fps) over that of the .58 rimfire and gave greater range and accuracy, though the muzzle energy was about the same (1,488 ft.-lbs.).