 The Model 1866 corrected problems encountered with the prototypical Model 1865, 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.It employed a robust version of the "trapdoor" breechblock design originated by Erskine S. Allin, Master Armorer of the Springfield Armory.Approximately 25,000 .58 caliber Springfield Model 1863 rifled muskets were converted by Springfield Armory for use by U.S. troops, the barrels being relined and rifled to .50 caliber and the trapdoor breech system affixed.