The Model 1866 Allin Conversion breech loading rifles were manufactured by Springfield Armory between 1867 and 1869. The Model 1866 rifle was a conversion of the Civil War production Model 1863 and Model 1864 Springfield rifle-muskets. The conversion involved fitting the rifle-musket with the Allin designed trapdoor breechblock, ejector and hammer, boring the .58 caliber barrel to a larger diameter, adding a .50 caliber liner and replacing the rear sight. Springfield Armory altered approximately 25,000 Springfield rifle-muskets to the Model 1866 configuration. The Model 1866 rifle was first used in action at "The Wagon Box Fight" against the Lakota near Fort Phil Kearny, Wyoming Territory, in 1869 where the sustained firepower of breech loading rifles enabled a small force of soldiers to defeat a superior number of warriors. The rifle has a National Armory Bright barrel, screw fastened barrel bands, trigger guard and buttplate. The hammer, lock plate and trigger retain the bright Civil War water quenched casehardened finish. The breechblock has the black, oil-quenched casehardened finish. The rear sight is blued. The black walnut stock is oil finished. The rifle has a cupped and slotted cleaning rod with threaded end and a folding leaf rear sight with 100, 300 and 500 yard apertures. The lock plate is dated "1864" behind the hammer and roll-stamped with the Springfield eagle and shield motif proceeded by "U.S./SPRINGFIELD" ahead of it. The top of the breechblock is dated "1866" and stamped with an eagle head inspection mark. "U.S." is stamped on the buttplate heel. The right side of each barrel band is stamped "U" pointing toward the muzzle. A final inspection mark consisting of three script initials in a rectangle with rounded ends is stamped on the left stock flat.

Extremely fine. 70% of the blackened case colors on the breechblock. The trigger, hammer and lock plate have 98% of the vivid water quenched case colors. The rear sight has 85% of the original blue finish. Nearly all of the National Armory Bright polish is present on the barrel, barrel bands, trigger guard, buttplate and cleaning rod. The stock is in very fine condition with sharp edges on the flats and barrel channel and shows only minimal storage wear. All of the markings, including the Ordnance inspection mark on the left flat are crisp. This is an outstanding example of a key U.S. martial longarm that would be difficult to improve upon.