The .44 Remington Magnum, or simply .44 Magnum (10.9×33mmR), and frequently .44 Mag, is a large-bore cartridge originally designed for revolvers. After its introduction, it was quickly adopted for carbines and rifles. Despite the “.44” designation, guns chambered for the .44 Magnum round, and its parent, the .44 Special, use 0.429 in (10.9 mm) diameter bullets.
The .44 Magnum is based on a lengthened .44 Special case, loaded to higher pressures for greater velocity (and thus, energy). [Wikipedia]