This model is similar to the Model 1865, except that in 1866, subsequent to field trials of the 1865 action, the arm underwent the following improvements: the caliber was reduced from .58 to .50 by boring out the barrel to .64 in. diameter and the insertion by brazing of a rifled .50 caliber liner tube into the bore.  The bolt was lengthened and a firing pin spring provided to keep the firing pin away from the face of the bolt.  The frail extractor was replaced by one operated by a strong, U-shaped spring.  The rim-fire cartridge was replaced by a center-fire, concealed primer cartridge using a 450 grain bullet and a 70 grain powder charge.  The breechblock pivoted on a screw, which was originally retained by a single slotted nut on the left side.  Problems in service led to eventual use of a two-part jam nut, which protruded slightly from the breechblock on the left side.  25,000 Civil War muzzle-loaders were directed to be altered to this Model 1866 improved Allin system.

     Our specimen was converted from a Model 1863.  The lock plate to the rear of the lock is marked “1864”.  In front of the lock is “US” “SPRINGFIELD” in two lines and an eagle.  Marked on top of the breech block is “1866” and eagle head.  The bands are marked “U”.  The top of the butt plate is marked “US”.  The left side of the barrel is marked “G”.  An inspector’s cartouche on the left side of the stock is faint “HWL”.  Note the original single slotted nut.  The bayonet is marked “US”.