Springfield Armory developed the "trapdoor" action through a directive from the Government to utilize as much of the current 58 Caliber Percussion musket as possible. This occurred in 1865. The "First Allin's Conversion" Model 1865 used nearly 45 individual parts and about 65 to 70 machining cuts to convert an 1863 58 Percussion musket into a 58 rim-fire breech-loader using the trapdoor action. There were 5000 of these conversions made in 1865. Tests indicated that the ballistics of eh 58 rimfire cartridge were not an improvement over the 58 percussion. A directive was issued to develop a 50 caliber cartridge and a strengthened and simplified trapdoor action still utilizing as much of the original percussion musket as possible. The "Second Allin's Conversion" Model 1866 was produced in a 50-70-450 caliber with about 25,000 were built in 1866. The 58 caliber barrel was bored out to 70 caliber and a 50 caliber sleeve was brazed inside, thus using the original musket barrel. The Model 1866 was the gun used at the Wagon-Box fight near Fort Phil Kearny in 1867. In 1868 Springfield designed a "Trapdoor" action with a 32" barrel in 50-70 and fitted it to a musket stock and lock, this was the Model 1868. This was followed by the Model 1870 which featured a slightly changed receiver withe same caliber and barrel length. The model 1873 used a similar receiver as the M1870 though now chambered in 45-70. I will mention that the Model 1870 was also produced in a carbine version with a 22" barrel. There was a Model 1867, Model 1868 and Model Cadet rifles produced.
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