The Springfield Model 1865, also named the First Allin (for its designer Erskine S. Allin) was a breechloading rifle, based heavily on the earlier Springfield Model 1863. The Model 1865 was also named a "Trapdoor Springfield" due to the breechblock mechanism it employed.
The Springfield Model 1865 was heavily based on the Model 1863 rifled musket, also produced by Springfield. Therefore the same stock and barrel were used on both firearms (although the barrel was shortened). Furthermore the Model 1865's breechloading mechanism was avaliable as a conversion for the Model 1863, which meant that the Model 1865 had a caliber of .58in (14.7mm).
The conversion from rifled musket to breechloader began with the breech section (of the barrel) being milled open, allowing for a hinged bolt to be fastened to the top of the barrel. The firing pin was then housed in the breechblock (which was fitted to the hinged bolt), before the extractor mechanism was fitted. Finally the hammer was flattened to strike the firing pin and a thumb catch fitted to open and close the breech.
The breechblock itself gave the name "Trapdoor Springfield" due to its hinge format. As the hinge was located at the top of the barrel it meant that the breechblock would swing downwards to close the breech, a system which resembles that of a trapdoor. This name also originates from the way in which the stock was cut to allow for the breechblock mechanism. The stock was left with a small wooden panel which slightly covered the mechiansm, which was usually taken off in military versions.Overall the conversion for the Springfield Model 1863 cost $5, considerably cheaper than the $20 it cost to buy a brand new Model 1865.
The Model 1865, however, suffered. Its breech lock system was extremely complicated when compared to the muzzle loading of the former muskets and the parts were put under a considerable amount of stress. This led to the Springfield Model 1866 being developed and entering production before the contract for the Model 1865 had been filled.
The Springfield Model 1865 was bored to accomodate a .58in (14.7mm) bullet in a cartridge. This was a relatively common caliber at the time, and was required for the conversion for the Model 1863, which was originally chambered for the .58in Minie Ball.
The rate of fire for the Model 1865 was expected to be around 8 to 10 rounds per minute, a great improvement on the Model 1863, which at best was expected to fire 4 shots in a minute. Furthermore soldiers were able to load the Model 1865 while prone (lying on the ground) which reduced the likelihood of being shot while reloading in long range combat, with less of the soldier in the open.
The Springfield Model 1865 was a popular conversion for the Model 1863, its higher rate of fire prompting the change. However it quickly became clear that the Model 1865 was not the best solution for a breechloading rifle, its mechanism being complicated and delicate at best. Therefore its successor, the Model 1866 was produced with a simpler mechanism and quickly replaced it in military circles.
The Springfield armory's stocks of the Model 1865 were sold to small American arms dealers, where the rifle was often shortened and stock thinned to reduce weight, improve manoeuvrability and appeal to the growing 'Cadet rifle' market. The Model 1865s (and their successors) were more popular than other breechloading rifles of the era, one example being the Joslyn Rifle which had been overshadowed thorugh disputes between the Joslyn Firearm Company and the US Government over quality. The Model 1865 was renowned for its quality even if the mechanism took years to refine.
The .58 and .50 Caliber Rifles and Carbines of the Springfield Armory" - Richard A. Hosmer, Published by North Cape Publications, May 2006