Although the term "sharpshooter" was used very loosly during the war, the regiment likely had rifled longarms. This would seem to be confirmed by your quote (presumably a period quote?) that they were "principally armed with Belgian RIFLES." Chances are if they were smoothbores they would not have been referred to as "rifles". Also, the Confederacy was much more particular about what they purchased abroad than the North was, and focused on more up to date firearms (i.e., rifled).

Most Belgian longarms were copies of French arms, and have the major characteristic of a back action lock. Arms with 2 barrel bands are properly called rifles, whereas those with 3 bands are rifle-muskets. However this strict definition was not always followed in period correspondence.

If I had to take a good guess I would say the reference is to a 2 band rifle fitted for a saber bayonet, as this was a very popular arm with "sharpshooter" regiments. Here's a picture of one from antiquearmsinc.

John Gross