Since no one has discussed this part of your OP I will - indeed, it should have many tales to tell!
These were hands down the most common and available shoulder arm on both sides in 1861. (In the very earliest engagements some units like the 20th Tennessee were even armed with unconverted flintlocks at Mill Springs/Logan's Crossroads in Kentucky in January, 1862.) Although they were steadily being replaced by more modern later models throughout the war some remained in service for the duration, though at the end mainly by home-guard or militia-type organizations. They remained in service longer in the Western and Trans-Mississippi Theaters than back East, particularly among Confederates, and were especially prominent at early war battles like First Bull Run/Manassas, Wilson's Creek, Pea Ridge, Fort Donelson, Shiloh, Perryville, and Stones River. In the North they fairly quickly became considered second-rate weapons and were often used to arm the first contingents of United States Colored Troops or USCT.