Pre-order a Mass Effect artbook for an in-game rifle on PC or Xbox
The "collector's rifle" refers to the Collector Assault Rifle, described by artbook publisher Dark Horse Comics as a gun whose "power source appears to be an internal organ with biotic capacitance; its ammunition resembles pellets of metallic enamel that strip shields off enemies with deadly efficiency." Strangely, Dark Horse also specifies that the codes work only on PC and Xbox 360, so PS3 owners will get nothing for their book purchase but the book they purchased.
[Thanks Mark!]