We ran an article on the Mississippis in our newsletter a few months ago... http://www.geocities.com/capitalguards/CGSDec06.pdf ... Whitney & Co. were selling these rifles to southern state governments all the way up thru the end of May, 1861, as well as the weapons that were already in the arsenals in the southern states. They are excellent combat weapons, something like the "AK-47" of their day and time. The Capital Guards had them in their armory pre-War, and Cleburne's "Yell Rifles" were armed with them, confiscated from a passing Mississippi steamboat.

The Whitney/Colt contract Springfields don't really start showing up in troop units until very late in 1862 or the first of 1863, and are an entirely different weapon. For what it's worth, Whitney had a contract with the British to make copies of the P1853 Enfield rifle for the Crimean War, and their copies of the Special Contract M1861 incorporate many little features of the Enfield.

Tom