Originally developed as a means of converting rifle muskets to breechloaders, the Allin modification ultimately became the basis for the definitive Springfield Model 1873, the first breech-loading rifle adopted by the United States War Department for manufacture and widespread issue to U.S. troops.

It employed a robust version of the trapdoor breechblock design originated by Erskine S. Allin, Master Armorer of the Springfield Armory.

The rifle was chambered for the powerful centerfire .50-70 Government cartridge (.50 caliber 450-grain (29 g) bullet; 70 grains (4.5 g) of black powder).

In the official 1867 government user booklet “Description and Rules for the Management of the Springfield Breech-Loading Rifle Musket, Model 1866”, the following is stated regarding a broken extractor and/or ejector: “It should be understood that the ejector and friction springs are convenient rather than necessary, and that the piece is not necessarily disabled if one or both of them should break, for the shell can be easily removed by the fingers after being loosened by the extractor hook.” Furthermore, the cleaning rod of the rifle can be used quite effectively to remove a stuck case in an emergency.

The rapid rate of fire that could be achieved disrupted the tactics of attacking Sioux and Cheyenne forces, who had faced muzzle-loading rifles during the Fetterman massacre only a few months before.