RE: 338 Ultra-mag why isn't it more popular?
The reason I think the .338 RUM is nor very popular is because it's hidden between the .338 Win. Mag., and the .340 Wby. Mag.
I really like the .338 calibers. One can own a .338-06 pushing a 250 gr. 2,600 f/s 3,752 ft/lb, up to a .338 Excaliber pushing a 250 gr. 3,250 f/s 5,862 ft/lb. Good luck.