Brass size, powder charge and velocity. Some, like the Winchester short mags and the Remington short action ultra mags, are short action cartridges that produce magnum velocities. Others, like the 300 H&H mag, were created by necking down the brass of large game calibers. The 300 H&H was created by necking down the brass of the 375 H&H. The 300 ultra mag and the 300 Weatherby mags are fairly large brass as well with large powder capacities.