|I've run across things saying the rifle was in use at least as early as Gettysburg while the carbine came into use later and that the carbine became more popular. Now I am checking both in the War of the Rebellion and have found something interesting on the carbine. Series 1, Volume 32, page 258 http://ebooks.library.cornell.edu/cgi/t/text/pageviewer-idx?c=moawar;cc=moawar;q1=Spencer%20carbine;rgn=full%20text;idno=waro0059;didno=waro0059;view=image;seq=0260 has in the conclusion of a letter to Stanton in April 1864 from General J.H. Wilson a discussion about the purchase of the carbines. It, the part of the letter I'm refering to, reads
The general desire of the best regiments is to be armed with the Spencer carbine. By arming one or two regiments in each department with them, their old arms turned in will supply the deficiencies in the other regiments
To the Second and Fourth Michigan, 700 Spencer carbines were furnished ; the Fourth United States, 600. It is proposed to furnish 700 for the Fifth Indiana and also the Second Iowa, of the Department of the Tennessee.