The Canning is a Weatherby Mk V, it's a hunting rifle. .300 Win Mag is also fairly rare as a sniping/marksman cartridge because it's comparitively quite expensive to .308 (which has less power but comparable ballistics) and 6.5mm cartridges (which have much flatter trajectories).
In fact, standard .300 Win Mag is considered inefficient and ineffective for sniping. The only loads in .300 WM used by the US Army are the MK248 Mod 0 and Mod 1. They are loaded extremely hot (68-71k PSI by SAAMI metrics) and use very heavy bullets (Mod 0 uses a 190gr boat-tailed hollowpoint with a windscreen for aerodynamics while Mod 1 uses a 220gr specialized match round). The brass is also reinforced to handle higher pressures and it cannot be used in anything other than specially strengthened .300 WM actions, which can only be found reliably in the M24 SWS (Sniper Weapon System). Furthermore, neither MK248 Mod 0 or Mod 1 can be purchased by civilians as they are on exclusive contract with the U.S. federal government for use by marksmen in the Army, Marine Corps, and a series of special operations units.
As an added note the "Canning" is a made-up name to avoid having to deal with copyright licensing fees from including an actual one to one Weatherby Mk V.