Check out a copy of the  Essential Guide book on Star Wars Weapons & Technology and you'll find reference to the E11 Carbine in there. All weapons and vehicles seen in First Strike have origins in some shape or form in the Star Wars universe, be that from the movies or the Expanded Universe (books, comics, computer/role-playing games). In some cases we may put our own spin on an asset to make it distinct to First Strike if there is room with the lore to manoeuvre, but it is all taken from the source.