The Springfield Model 1866 is famous for the impact it made on the frontier during Red Cloud’s War due to the fact that the rate of fire was vastly superior to the standard muzzleloading rifles the Native Americans were accustomed to facing in battle. The bonus of an almost tripled rate of fire in addition to being able to load the rifle while laying in a prone position negated the commonly effective Indian tactic of drawing a volley of fire with distractions before charging with superior numbers to overwhelm the slower loading musket equipped soldiers.
“Originally developed as a means of converting rifled muskets to breechloaders, the  ultimately became the basis for the definitive Model 1873, the first breech-loading rifle adopted by the United States War Department for manufacture and widespread issue to U.S. troops.”
Specifically this weapon saw its most significant action during two skirmishes along the Bozeman Trail: The Hayfield Fight and the Wagon Box Fight, where forces vastly outnumbered repelled attacking Natives with the aid of this tool. It is amazing how the slightest adjustments to standard weapons can make such an impact on the battlefield.
An interesting thing for me personally is that my father was given one of these rifles as a gift from a family friend. I don’t know if he has ever attempted to fire it or not (though I would imagine not, because of its age; though his might be the ’73 model).