Photo, courtesy of Jakub of imfdb.org
The Bravo-51 is most-often chambered in 7.62x51mm NATO (.308 Winchester), with a reasonably flat trajectory for engaging within 300 ~ 700 meters (in my opinion), and a short case designed for reliable feeding in a short action. .300 Winchester Magnum variants are also available. They're outfitted with match grade heavy barrels (457mm, 508mm, 559mm, or 610mm) and threaded for a suppressor, in a fiberglass McHale stock with aluminum pillars, Harris bipod, scope mounts / Picatinny rail, and sling.
Most just refer to it as Bravo-51, if anything at all. From the Marine friends I've asked, they hadn't ever seen one deployed, but apparently Sniper Platoons were trained in their use. It seemed to be more associated with the Army, within which I (regrettably) haven't yet any friends to ask. NCIS writers seem to have employed a little artistic license, and tenuously basing the "Kate" mythology on a US Army National Guard sharpshooter who runs the Sniper Central webpage. When he served, he was issued an Army M24 SWS, he had named Kate (pic) - a common tradition. After serving, he ordered himself a Spec-Ops Bravo-51, which he dubbed Kate II - presumably to honor his original M24, Kate, with which his Bravo-51 shares many similarities. Besides this, I'm not aware of any other connections to the Bravo-51 rifle, and the nickname Kate.