From The Manufacturer/Wikipedia/People Smarter Than Us:
The 338 Marlin Express is the most lethal combination of flat trajectory and game-leveling energy to ever grace a traditional lever gun. Truly the ultimate choice for big game out to long range, it’s yet another proud ballistic achievement from the company that brought you the 308 Marlin Express, which mirrors the 308 Win. Together, the 338 Marlin Express and 308 Marlin Express are a deadly one-two punch capable of taking any game animal in North America. Even more notably, they’re the only 300-yard chamberings for a traditional, tubular-magazine lever-action rifle, and you’ll only get them from Marlin.