Springfield Model 1866
Overview
 
The Springfield Model 1866 was the second iteration of the Allin-designed trapdoor breech-loading
Breech-loading weapon
A breech-loading weapon is a firearm in which the cartridge or shell is inserted or loaded into a chamber integral to the rear portion of a barrel....

 mechanism. Originally developed as a means of converting rifled musket
Rifled musket
The term rifled musket or rifle musket refers to a specific type of weapon made in the mid-19th century. Originally the term referred only to muskets that had been produced as a smoothbore weapon and later had their barrels rifled...

s to breechloaders, the Allin modification ultimately became the basis for the definitive Model 1873
Springfield Model 1873
The Model 1873 "Trapdoor" Springfield was the first standard-issued breech-loading rifle adopted by the United States Army...

, the first breech-loading rifle adopted by the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 War Department for manufacture and widespread issue to U.S. troops.

The Model 1866 corrected problems encountered with the prototypical Model 1865
Springfield Model 1865
The Springfield Model 1865 was an early breech-loading modification of the Springfield rifle musket design.During the U.S. Civil War, the advantage of breech loading rifles became obvious. The rifled muskets used during the war had a rate of fire of 3 or 4 rounds per minute...

, in particular a simplified and improved extractor and a superior .50 caliber centerfire cartridge (the Model 1865 used a .58 caliber rimfire cartridge with mediocre ballistics), among many other less significant changes.
 
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