Companies "A" and "B" were issued the M1841 Rifled Musket (AKA Mississippi Rifle), cal .54.
http://www.collegehillarsenal.com/shop/product.php?productid=695

Other companies carried the "George Law" percussion musket. Information found in paragraph labeled Equipment comes from the book by Col Oates.
http://en.wikipedia.org/wiki/15th_Regiment_Alabama_Infantry