First of all,there is no 30-30 marlin,no 308 remington,and no 30-06 remington.There are Marlin guns chambered in 30-30 winchester,and remington guns chambered in 308win and 30-06 springfield.Secondly,the USA includes Alaska so if the gun isn't suitable to kill Alaskan brown bears,it can't be the best cartrtridge choice for the USA.Therefore my choice is the 300winmag.By the way,all of your choices are cartridges,not calibers.